crimson/osd: Remove head memeber from ObjectContext

crimson/osd: Remove head memeber from ObjectContext

Before this patch, ObjectContext had a head member which was used
to get the head obc of a clone object.
This member caused the head object to being referenced while
attempting to 'clear_replica_obc' (Since we only evict un-referenced
obc from the obc_registery).
This mechanism, of obtaining the head, is no longer needed since
'with_clone_obc' loads the head object context first (using
oid.get_head).

In this commit, head is removed from ObjectContext class and users
are removed as well.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>