ReplicatedPG::submit_push_complete don't remove the head object

The object would have had to have been removed already.  With
fd caching, this extra remove might check the wrong replay_guard
since the fd caching mechanism assumes that between any operation
on an hobject_t oid and a remove operation, all operations on that
hobject_t must refer to the same inode.

Signed-off-by: Samuel Just <sam.just@inktank.com>