xdcr: dropped work_seq_to_snapshot_seq

It's not being used anymore

Change-Id: I4a14aa192a997f012c3c06ed03a8cc90d63ed58a
Reviewed-on: http://review.couchbase.org/38445
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>