ReplicatedPG: don't count deletions as ops

ReplicatedPG: don't count deletions as ops

Counting them as ops but not requeueing the pg for recovery causes
backfill to stall when only deletions are sent in
recover_backfill(). Deletions are cheap and don't need to be acked, so
we can simply stop counting them as ops.

Fixes: #2044
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>