Remove ~# from surface syntax

For some reason, it seems that the `ConstraintKinds` commit
introduced `~#` into Haskell syntax, in a pretty broken manner.
Unless and until we have an actual story for unboxed equality,
it doesn't make sense to expose it. Moreover, the way it was
donet was wrong enough and small enough that it will probably be
easier to start over if we do that. Yank it out.

Reviewers: bgamari, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: RyanGlScott, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #15209

Differential Revision: https://phabricator.haskell.org/D4763