Revert "RollbackAction: Implement AJAX interface and require POST"

Revert "RollbackAction: Implement AJAX interface and require POST"

This partially reverts commit 9af38c046c.

* The new JS modules continue to exist. While not used by default,
  the mediawiki.api.rollback may be used elsewhere by now.
  The mediawiki.page.rollback module may still be used on cached
  pages and should continue to work.

* The new API behaviour remains.

* To avoid breaking mid-air rollbacks again, the token remains
  the same (plain "rollback" salt). We're not re-introducing the
  web-specific salt again.

Bug: T136375
Change-Id: Iba17ce55ff9506e838bfc6e70ca280e5a20b77b6