Revert UMD for now, due to #13812.

Revert UMD (#13772 & friends) for now, due to #13812.

We'll hopefully be able to revert this reversion and land a fully-working version of UMD in v3.3.0, after we figure out a solution for #13812. But right now, we just want to get v3.2.0 shipped already, since it's late, and UMD is our sole blocker at this point.

To: @fat @XhmikosR for review.

For less confusion, I recommend reading the whitespace-ignorant diff:
https://github.com/twbs/bootstrap/pull/13904/files?w=1