Revert "mediawiki.mixins: Add border-radius mixin"

Prefixes for border-radius haven't been needed in a long time. This
degrades gracefully and isn't worth the overhead.

In addition, only a few weeks ago did we merge commits that removed all
this prefixes (e.g. in Ie5ce3202b4). Adding this mixin would encourage
doing the reverse.

This reverts commit 591ba359f283c5ab4ba1b2a0359a98692a3b204b.

Change-Id: Ic25a001de3fe1516cc4939b3159e362e87cb790b
Merge "Revert "mediawiki.mixins: Add border-radius mixin""