remove test for T -> ! coercion

We used to allow `T -> !` coercions (yes!! not `! -> T`) in unreachable
code. This was later removed during 2018 stabilization attempt, see:
- #40800
- 59688e1
- #46325

I've kept `tests/ui/coercion/coerce-to-bang-cast.rs`, as that is a
reasonable test for us *not* having `-> !` coercions.

Author: WaffleLapkin