Remove 'prefsnologin' message, don't use 'watchnologin' where inappropriate

There is no reason why the default can't be used instead.
No other special pages requiring login have special messages for the
title, as far as I know.

Left one use of 'watchnologin' in WatchAction alone, since it also uses
another special message there. This message is also currently used by
the MobileFrontend extension.

Change-Id: I7878ed3692358cee1f5785b34ab48a0cc83c05bc