BP_Activity_Activity::delete_activity_meta_entries doesn't clear cache

[DJPaul]

BP_Activity_Activity::delete_activity_meta_entries() clears items from the activity meta table, but not from the wp_cache. This method is used in BP_Activity_Activity->delete().

To recreate: add an activity item, set some meta, use BP_Activity_Activity::delete_activity_meta_entries(), and then do bp_activity_get_meta(). The DB row will have been correctly deleted, but you'll get the value back from the cache.

Unit test is: BP_Tests_Activity_Class::test_delete_activity_meta_entries

[boonebgorges]

Closing in favor of #4804, which has a patch.