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.
Merges [32852] to the 4.2 branch. Fixes #31388 for 4.2

Built from https://develop.svn.wordpress.org/branches/4.2@32853


git-svn-id: http://core.svn.wordpress.org/branches/4.2@32824 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/upgrade.php

@@ -2457,11 +2457,6 @@ 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() ) {