Summary:
Previously, I added an ad hoc check for unboxed tuples and
sums in standalone-derived instances to fix #12512, under the
pretense that polymorphism over `UnboxedTupleRep` and
`UnboxedSumRep` was a lie. But that is no longer the case, and so
this ad hoc check can be removed entirely. Less code, and easier to
understand.

Test Plan: make test TEST=T12512

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4271