OSD: no need to remove snapdirs on _remove_pg()

The snapmapper patches removed snapdirs altogether.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,OSD: _remove_pg must remove pg keys

Instead of doing this in OSD::_remove_pg, pass a transaction
to on_removal and do it in PG.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge branch 'wip_4860' into next

Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>