Revert "ReplicatedPG::get_snapset_context: assert snap obj is not missing"

This breaks mark_lost_unfound_revert.

This reverts commit 0d2177a18071ad9c9581826a43751c36bab5b2db.
Merge pull request #1695 from ceph/wip-8153

Revert "ReplicatedPG::get_snapset_context: assert snap obj is not missin...

Reviewed-by: Sage Weil <sage@inktank.com>