Deprecate "Avoid page_touched update for HTTP GET action=purge requests"

Deprecate "Avoid page_touched update for HTTP GET action=purge requests"

This reverts most of commit c84ba4d86420d7af918e572e2cd4613d7be185b3.
The changes made there are no longer needed, and nothing in Gerrit seems
to have started using them since they were added.

The added constants in WikiPage, WikiPage::getLastPurgeTimestamp(), and
Article::getLastPurgeTimestamp() are deprecated, useless, and unused,
but not removed yet since they snuck into 1.28 so we can't revert them
without a deprecation period. Sigh.

Bug: T145649
Change-Id: I526fd4e004bee84c831a4cee71e44e92ee73480b