rgw: fix get_system_obj() caching

Originally this was get_obj(), but in the cleanup work this was changed
to get_system_obj() with somewhat different params. Updated cache to use
new interface.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: fix caching for remove_system_obj()

Align the RGWCache method with the RGWRados method. Also, don't use obj
head removal objclass call, as it's for data objects, not metadata.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>