Do not generate given kind-equalities (fix Trac #8566)

This is a long-standing bug.  We were generating a Given
equality between kind variables, and (at least until we support
kind coercions) we can't do that.

The fix is to drop such Given equalities entirely. That may
mean we can't prove some things, but that's fair enough -- the
current proof language can't express such proofs.

See Note [Do not create Given kind equalities] in TcSMonad
Better debug printing
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