Cleanup: drop Auth::User::proxy_auth_list header cache

Cleanup: drop Auth::User::proxy_auth_list header cache

This list/cache was originally used to short-circuit auth helper lookups
based on previousy seen [Proxy-]Authorization header strings.
However, that permitted replay attacks in most auth schemes and has been
replaced by scheme-specific mechanisms:

* Basic and Digest credentials are cached in the global user name cache
  wih additional nonce/password comparisons to verify located entries.

* NTLM and Negotiate credentials are cached in the ConnStateData with
  exact-match comparison done to verify tokens.