Remove a redundant index drop, it'll be dropped and re-created a few lines further down.

Fixes a warning during updating from 4.0 or earlier.
Fixes #31388 for trunk


git-svn-id: https://develop.svn.wordpress.org/trunk@32852 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/upgrade.php

@@ -2620,11 +2620,6 @@ function pre_schema_upgrade() {
 		}
 	}
 
-	if ( $wp_current_db_version < 30133 ) {
-		// dbDelta() can recreate but can't drop the index.
-		$wpdb->query( "ALTER TABLE $wpdb->terms DROP INDEX slug" );
-	}
-
 	// Upgrade versions prior to 4.2.
 	if ( $wp_current_db_version < 31351 ) {
 		if ( ! is_multisite() ) {