commonElements: Move sup, sub styling from Vector's CSS to common styles

This really should be a basic style, not limited to one skin. All
Wikimedia wikis which used this hack seem to have it on their
common.css, not vector.css.

Follow-up to Ic5ba8363.

Bug: 49965
Change-Id: Ie98f8b57a71b703fbba19c201c13edef3f6adc55
Merge "commonElements: Move sup, sub styling from Vector's CSS to common styles"