os/bluestore: Split BlueStore::_do_remove()

Split BlueStore::_do_remove into 2 parts:
- part that removes object (in _do_remove)
- part that processes head object and tries to unshare blobs
  (new function _maybe_unshare_on_remove)

No logic changes.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>