How do I remove or disable “Or link to existing content” in “insert link” dialogue?

We could hook into the after_wp_tiny_mce with some CSS to hide it, if the wplink editor plugin is loaded.

Example:

add_action( 'after_wp_tiny_mce', function( $settings )
{
    // Check for the 'wplink' editor plugin
    if(    isset( $settings['content']['plugins'] ) 
        && false !== strpos( $settings['content']['plugins'], 'wplink' ) 
    )
        echo '<style>
                  #link-selector > .howto, #link-selector > #search-panel { display:none; }
        </style>';
} );