Unmute testCacheUnderConcurrentAccess (#91186)

This test has been muted for a very long time. I haven't been able to
reproduce the failure, so I'm going to unmute it and see if it starts
failing on CI.

Relates https://github.com/elastic/elasticsearch/issues/51914

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/security/authz/accesscontrol/DocumentSubsetBitsetCacheTests.java

@@ -359,7 +359,6 @@ public class DocumentSubsetBitsetCacheTests extends ESTestCase {
         });
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/51914")
     public void testCacheUnderConcurrentAccess() throws Exception {
         // This value is based on the internal implementation details of lucene's FixedBitSet
         // If the implementation changes, this can be safely updated to match the new ram usage for a single bitset