Delete custom IE-fixes.css

Our custom CSS is a 1:1 copy of upstream from before 12/2009:
http://bzr.mozilla.org/bugzilla/4.2/view/5474/skins/standard/IE-fixes.css

There is no sense in keeping our copy, since there have been no
changes in the meantime to definitions which already existed in
our old revision, plus for new definitions added since 2009 we
fall back to the default definitions anyway.

Bug: 54823
Change-Id: I64824ad90d07ec68f9c086acc391cf6147b99bc1
Merge "Delete custom IE-fixes.css"