Local system tables from `system` namespace use LocalStrategy
replication, so they do not need to be concerned about gc grace
period. Some system tables already set gc grace period to 0,
but other ones, including system.large_partitions, did not.
That may result in millions of tombstones being needlessly
kept for these tables, which can cause read timeouts.
Fixes#6325
Tests: unit(dev), local(running cqlsh and playing with system tables)