Monobook: Remove unused file FF2Fixes.css

As of Id8214c6a5 and Ia54dd738b (per bug 35906) we started
blacklisting Firefox 2 from the additional resource loader.

Though we haven't supported Firefox 2 for several years, Monobook
has had this patch for years and we never removed it (cheap to
maintain).

Since wikibits.js is no longer being loaded in Firefox 2, this
fix is now obsolete. It is never loaded, not even in Firefox 2.

It is causing problems because the legacy code incorrectly
detects Firefox 20 as Firefox 2, so instead of being a useless
file that is never loaded, it is actually causing problems by
being loaded in Firefox 20.

Bug: 47202
Change-Id: I2fdd0da8c17553a3465ef27c46a4632e135c4405