Revert r44528 "(bug 1941) Add textarea { font-family: monospace; } to shared.css...

Revert r44528 "(bug 1941) Add textarea { font-family: monospace; } to shared.css for Safari"

Demonstrable regression for Lingala, per
<https://bugzilla.wikimedia.org/show_bug.cgi?id=1941#c16>, and
plausibly for some other poorly-supported scripts as well.  Also
harmed some other things:

* Can make it hard to work with multiple scripts generally, since
unusual characters may fall back to proportional fonts for some
characters.
* It's much harder to tell apart some kinds of similar-looking
characters like dashes and hyphens.

Plus a lot of people think it's ugly.  Browser consistency shouldn't be
a goal here per se -- different browsers/platforms have different
display conventions, and we should respect that where possible unless it
causes demonstrable problems (which this hasn't AFAICT).