Do not unbreak alternative service in ClearAlternativeService.

Do not unbreak alternative service in ClearAlternativeService.

Do not unbreak alternative service in
HttpServerProperties::ClearAlternativeService().

* This is wrong, because only because an alternative service is not mapped to
  one origin any longer, if it was broken, it remains broken and should not be
  used for other origins.
* QuicStreamFactory::ProcessGoingAwaySession() used to call
  ClearAlternativeService(), and right after that, SetAlternativeService, to
  mark an alternative service recently broken.  Now QuicStreamFactory uses
  MarkAlternativeServiceRecentlyBroken calls instead, so this behavior in
  ClearAlternativeService is not required any more.
* Note that despite what the TODO comment says, broken alternative services
  should remain in the mapping (until explicitly cleared by
  ClearAlternativeService).

BUG=392575

Review URL: https://codereview.chromium.org/1198363003

Cr-Commit-Position: refs/heads/master@{#335907}