Currently the code in server controller is using a periodic SQL poll loop to inspect the service mode of runnable tenants.

Since #106414 was merged, we now have a rangefeed-enabled cache of this state. So the SQL polling loop can be removed.

As a side benefit, this also makes it possible for the server controller to identify servers by tenant ID instead of by name.

Epic: CRDB-26691

Jira issue: CRDB-30208