Source

File: wp-admin/includes/upgrade.php

function drop_index( $table, $index ) {
	global $wpdb;

	$wpdb->hide_errors();

	$wpdb->query( "ALTER TABLE `$table` DROP INDEX `$index`" );

	// Now we need to take out all the extra ones we may have created.
	for ( $i = 0; $i < 25; $i++ ) {
		$wpdb->query( "ALTER TABLE `$table` DROP INDEX `{$index}_$i`" );
	}

	$wpdb->show_errors();

	return true;
}