crimson/osd: fix PGBackend::remove() to return ENOENT on no-op deletes

crimson/osd: fix PGBackend::remove() to return ENOENT on no-op deletes

PGBackend::remove() was returning success when asked to delete a
non-existent object or an already-whiteout object that must remain
a whiteout. The classic OSD returns -ENOENT in both cases. Fix both
paths to return enoent, and remove the duplicate !os.exists check.

Fixes: https://tracker.ceph.com/issues/76529
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>