mediawiki::packages: remove libtidy-0.99-0

We shell out to the tidy standalone binary, which is in the 'tidy' package.
(Which depends on libtidy-0.99-0 anyway, but that's besides the point; the
point is that this package is not an actual top-level requirement for MediaWiki
and should therefore not be listed here.)

Change-Id: Ie3c6173c90d757aef31ccad6b1365294a9caa011