Commit ddc3b6dcf5 added a check of group0 state in
get_schema_for_write(), but group0 client can only be used on shard 0,
and get_schema_for_write() can be called on any shard, so we cannot use
_group0_client there directly. Move assert where we use another group0
function already where it is guarantied to run on shard 0.
Closesscylladb/scylladb#25204
198cfc6fe7