Remove mediawiki stylesheet hack

According to https://bugs.webkit.org/show_bug.cgi?id=28350#c46,
we needed to wait until most of the top mediawiki installations
have to switched to version 1.5 or above. This time has come as
there is only one wiki in the top 200 with a version below 1.5.

Review URL: https://codereview.chromium.org/63383003

git-svn-id: svn://svn.chromium.org/blink/trunk@161558 bbb929c8-8fbe-4397-9dbb-9b2b20218538