counters: drop tombstone handling in transform update to shards
counters: drop tombstone handling in transform update to shards

Encountering tombstones while transforming counter update from deltas to
shards is expected to be rare due to the fact that counter cells cannot
be recreated once removed.
This assumption makes it unnecessary to care much about removed cells
during delta->shard transformation as it adds complexity to the code and
is not required to produce correct results.

avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla