osd: move pg removal flag into a static helper

Signed-off-by: Sage Weil <sage@redhat.com>
ceph_objectstore_tool: use '_remove' pgmeta omap key to indicate removal too

We could conceivably mark the PG for backfill, but to do this correctly
we have to do the log rollback, which uses various methods in PG to update
snap metadata that we simply don't have access to from this tool.

Signed-off-by: Sage Weil <sage@redhat.com>