mediawiki.user: Deprecate #bucket

Rationale:
- On the Wikimedia cluster, it is only used by AFT, which is moribund.
- It's too complicated for what it is.
- Its usage leads to a proliferation of large, persistent cookies that
  degrade site performance.
- If it were rewritten to be simpler, more generic, and cookie-free,
  it would still be an odd fit for the user module.

Change-Id: I5a66e1a4fb60f6c67d1fb5312ccb629bb7964053