CSS: Don't workaround the IE 11 iframe-in-fullscreen sizing issues

CSS: Don't workaround the IE 11 iframe-in-fullscreen sizing issues

IE 11 used to have an issue where if an element inside an iframe was put
in fullscreen mode, the element dimensions started being 100 times too small;
we've added a workaround that would multiply them by 100. However, the IE 11
issue has been unexpectedly fixed and since our detection was really detecting
the browser and not a bug, we've started breaking the browser instead of fixing
it.

Since there's no good way to detect if the bug exists, we have to back the
workaround out completely.

Fixes gh-3041
Refs gh-1764
Refs gh-2401
Refs 90d828bad0d6d318d73d6cf6209d9dc7ac13878c