Get rid of Internet Explorer for Mac fixes in Monobook's main.css.

Wikipedia says that IEMac's initial release was 17 years ago and final
release 10 years ago.

IEMacFixes.css, which was basically unmaintained after r2909 (which was
done to ensure HTML validation compliance), was deleted in r61787.

Moved some rules which apparently still apply to IE6 to IE6Fixes.css, as
per code review.

These fixes were originally added in the following revisions (incomplete
list):
* r2909
* r3165
* r3181
* r3658
* r3765

Change-Id: I2352be07964060bc5ed7b3bd82007755b3168654
Merge "Get rid of Internet Explorer for Mac fixes in Monobook's main.css."