db: remove unused belongs_to_{current,other}_shard(s) functions
db: remove unused belongs_to_{current,other}_shard(s) functions

Obsoleted by new sharding mechanism, but break the build for some.

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