remove_meta_box removes the whole Categories box.
The Most used tab don’t have any hook.
Don’t modify WordPress core files, there’s always a way to modify WP behavior without touching the core.
/**
* Place the script in the theme's functions.php file
*/
add_action( 'admin_head-post-new.php', 'wpse_70874_hide_most_used_cats' );
add_action( 'admin_head-post.php', 'wpse_70874_hide_most_used_cats' );
function wpse_70874_hide_most_used_cats()
{
// This function runs in all post types (posts, pages and cpts)
// So we need to check for the correct type
global $post;
if( 'post' != $post->post_type )
return;
// If checking for a custom taxonomy, the <ul> id is 'custom-taxonomy-slug-tabs'
// See /wp-admin/includes/meta-boxes.php, line 322, WP 3.4.2
?>
<style type="text/css">
#category-tabs .hide-if-no-js {display:none;}
</style>
<?php
}