Jose, give this a shot and see if that’s the kind of thing you had in mind..

EDIT: Done a little bit of testing and copying of core code to archieve this, but i think the new example i’ve added below should do what you’ve described, remove the buttons, “Insert into Post”, etc…, but keep a delete link. I’ve left the original example i provided because it may be useful to someone.

New Example

Remove buttons, but keep a delete link

function myAttachmentFields($form_fields, $post) {
    // Can now see $post becaue the filter accepts two args, as defined in the add_fitler
    if ( substr( $post->post_mime_type, 0, 5 ) == 'image' ) {
        $form_fields['image_alt']['value'] = '';
        $form_fields['image_alt']['input'] = 'hidden';

        $form_fields['post_excerpt']['value'] = '';
        $form_fields['post_excerpt']['input'] = 'hidden';

        $form_fields['post_content']['value'] = '';
        $form_fields['post_content']['input'] = 'hidden';

        $form_fields['url']['value'] = '';
        $form_fields['url']['input'] = 'hidden';

        $form_fields['align']['value'] = 'aligncenter';
        $form_fields['align']['input'] = 'hidden';

        $form_fields['image-size']['value'] = 'thumbnail';
        $form_fields['image-size']['input'] = 'hidden';

        $form_fields['image-caption']['value'] = 'caption';
        $form_fields['image-caption']['input'] = 'hidden';

        $form_fields['buttons'] = array(
            'label' => '',
            'value' => '',
            'input' => 'html'
        );
        $filename = basename( $post->guid );
        $attachment_id = $post->ID;
        if ( current_user_can( 'delete_post', $attachment_id ) ) {
            if ( !EMPTY_TRASH_DAYS ) {
                $form_fields['buttons']['html'] = "<a href="" . wp_nonce_url( "post.php?action=delete&amp;post=$attachment_id", "delete-attachment_' . $attachment_id ) . "' id='del[$attachment_id]' class="delete">" . __( 'Delete Permanently' ) . '</a>';
            } elseif ( !MEDIA_TRASH ) {
                $form_fields['buttons']['html'] = "<a href="#" class="del-link" onclick=\"document.getElementById('del_attachment_$attachment_id').style.display='block';return false;\">" . __( 'Delete' ) . "</a>
                         <div id='del_attachment_$attachment_id' class="del-attachment" style="display:none;">" . sprintf( __( 'You are about to delete <strong>%s</strong>.' ), $filename ) . "
                         <a href="" . wp_nonce_url( "post.php?action=delete&amp;post=$attachment_id", "delete-attachment_' . $attachment_id ) . "' id='del[$attachment_id]' class="button">" . __( 'Continue' ) . "</a>
                         <a href="#" class="button" onclick=\"this.parentNode.style.display='none';return false;\">" . __( 'Cancel' ) . "</a>
                         </div>";
            } else {
                $form_fields['buttons']['html'] = "<a href="" . wp_nonce_url( "post.php?action=trash&amp;post=$attachment_id", "trash-attachment_' . $attachment_id ) . "' id='del[$attachment_id]' class="delete">" . __( 'Move to Trash' ) . "</a><a href="" . wp_nonce_url( "post.php?action=untrash&amp;post=$attachment_id", "untrash-attachment_' . $attachment_id ) . "' id='undo[$attachment_id]' class="undo hidden">" . __( 'Undo' ) . "</a>";
            }
        }
        else {
            $form_fields['buttons']['html'] = '';
        }
    }
    return $form_fields;
}
// Hook on after priority 10, because WordPress adds a couple of filters to the same hook - added accepted args(2)
add_filter('attachment_fields_to_edit', 'myAttachmentFields', 11, 2 );

Original Example

Example of how to remove all buttons, but provide an insert into post button.

function myAttachmentFields($form_fields, $post) {

    if ( substr( $post->post_mime_type, 0, 5 ) == 'image' ) {

        // .. $form_fields code here(per above example), trimmed for illustration ..

        $form_fields['buttons'] = array(
            'label' => '', // Put a label in?
            'value' => '', // Doesn't need one
            'html' => "<input type="submit" class="button" name="send[$post->ID]" value="" . esc_attr__( "Insert into Post' ) . "' />",
            'input' => 'html'
        );
    }
    return $form_fields;
}
// Hook on after priority 10, because WordPress adds a couple of filters to the same hook
add_filter('attachment_fields_to_edit', 'myAttachmentFields', 15, 2 );

Set the filter to priorty 15, so it’s hooked on after WordPress has hooked it’s own filters on(because it adds them at the default priority of 10).

Not sure you wanted the insert button, just wanted to give an example of replacing the original buttons with a singular insert one..

Hope that helps..