Cache API: Remove unused usermeta global cache group.

The `usermeta` group was introduced in WP 2.0, but was never actually used in WordPress core.

The `user_meta` group is used instead.

Follow-up to [3011], [15482], [16562], [53823], [53832], [54940].

Props ignatggeorgiev, peterwilsoncc, oglekler, pamprn09, spacedmonkey, SergeyBiryukov.
Fixes #58175.
Built from https://develop.svn.wordpress.org/trunk@55940


git-svn-id: http://core.svn.wordpress.org/trunk@55452 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/load.php

@@ -772,10 +772,9 @@ function wp_start_object_cache() {
 				'rss',
 				'users',
 				'user-queries',
+				'user_meta',
 				'useremail',
 				'userlogins',
-				'usermeta',
-				'user_meta',
 				'userslugs',
 			)
 		);