This PR is a part of
https://github.com/paritytech/polkadot/issues/3211.

This PR prepares ground for the following runtime changes required for
PVF pre-checking. Specifically, we do several changes here:

1. We remove `validation_code_at` and `validation_code_hash_at`. Those
   functions are not used. They were added in the early days with intent
   to use it later but turned out that we do not need them.
2. We replace `validation_code_hash_at` with just `current_code_hash`
   for the case of inclusion and candidate checking.
3. We also replace `last_code_upgrade` with a direct query into
   `FutureCodeHash` and `UpgradeRestrictionSignal`. Those in conjunction
   should replace the logic that was used for allowing/disallowing
   upgrades. This requires special attention of the reviewers.
4. Then we remove the machinery required to support those queries.
   Specifically the code related to `UseCodeAt`. We do not need it since
   we do not answer the historical queries. However, we still leave all
   the data on-chain. At some point we may clean it up, but that would
   be needed to be done with a dedicated migration which can be done as
   follow-up.
5. Some now irrelevant tests were removed and/or adapted.