In bp_activity_delete_meta(), clear entire activity item cache when deleting all meta

This fix will become moot after migrating to WP metadata API, but I'm fixing
it just to get the unit tests to pass in the meantime :)

See #4551

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7852 cdf35c40-ae34-48e0-9cc9-0c9da1808c22