Remove untyped methods from activerecord.rbi (#347)

No reason to have all these methods, they only make maintaining
forward/backward compatibility more difficult for no benefit.

If you want to know all the methods that exist in ActiveRecord, you
should be generating gem RBIs with Sorbet or Tapioca.

I've left in mixes_in_class_methods usages, to make sure that this
commit doesn't remove anything
that `srb rbi gems` wouldn't know about itself.

This resolves a lot of issues with the RBI (e.g. different arguments
between these sigs and the actual methods in rails).