Rip out --enable-bufferevents

Periodically we run across packagers that provide the bufferevents shotgun and aim it at the user's foot: first was gentoo that turned it on by default, and today was freebsd that gives you a tantalizing option to click without any hint about how you'll be missing a foot afterwards.

I proposed in #18189 (moved) that we rip out the ability to set --enable-bufferevents, so we end the steady trickle of users who lose feet. We should totally do this.

The next question then is: should we also remove all the code that will effectively become dead code then? That one I want Nick's opinion on.