Revert "ReplicatedPG::recover_backfill: adjust last_backfill to HEAD if snapdir"

This reverts commit 545135f3e1c75318940caa2c82ac32a53fc4f957.

Conflicts:

	src/osd/ReplicatedPG.cc
Merge branch 'wip-6685-firefly' into firefly

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