Garbage collect unused chacha20 code

Two copies of chacha20 were imported into the tree on Apr 15 2017 (r316982)
and Apr 16 2017 (r317015). Only the latter is actually used by anything, so
just go ahead and garbage collect the unused version while it's still only
in CURRENT.

I'm not making any judgement on which implementation is better. If I pulled
the wrong one, feel free to swap the existing implementation out and replace
it with the other code (conforming to the API that actually gets used in
randomdev, of course). We only need one generic implementation.

Sponsored by: Dell EMC Isilon