bits: remove /w/ symlink, add a /w/404.php symlink

bits isn't a valid hostname for MediaWiki, so regular entry points just
fail and log fatals. Remove the /w/ symlink and symlink just 404.php
instead, which was the intention of Ida784b all along.

Bug: 54805
Change-Id: Ia73fca2012696f7021cc8fb75da527654b83e088