objectcache: Remove "for b/c" from internal ObjectCacheFactory comment

Follows-up 7475063bfd. We don't usually keep back-compat between
a subject and its own tests. It looks like the tests don't have
an alternative way to set or test this, so perhaps this is not for
back-compat but rather for internal usage in tests.

Change-Id: I4d212433326592589a45d71a081c4d01377e689b

includes/objectcache/ObjectCacheFactory.php

@@ -90,7 +90,7 @@ class ObjectCacheFactory {
 	/** @var callable */
 	private $dbLoadBalancerFactory;
 	/**
-	 * @internal ObjectCacheTest only, needed for b/c.
+	 * @internal ObjectCacheTest only
 	 * @var string
 	 */
 	public static $localServerCacheClass;