crimson/osd/pg_backend: remove stopping and peering

crimson/osd/pg_backend: remove stopping and peering

These two state variables duplicate checks that *should* already
be handled by the IOInterruptCondition.  None of the stopping
checks should ever trigger because the caller would be in an
interruptible future context which already performed that check.

Moreover, peering doesn't really work -- it relies on the callback
firing prior the call to on_activate_complete(), and there isn't any
guarantee that will happen.  Storing the epoch from when the
callback was created as we do in IOInterruptCondition
would be required.

Signed-off-by: Samuel Just <sjust@redhat.com>