Revert "Add API action=watch 'entirewatchlist' param to bulk-clear watchlist."

Unbounded modification = replication lag. And this function is specifically intended for clearing very large watchlists which would easily escalate to pageable lags of several minutes.

This reverts commit 66b07ea38133992f98bb158c95103b720fe958ec.

Change-Id: Ie34be712884407e671d715e56bda3bff4a7e78f4