Comments only -- clarifying Notes around compatibility.
Remove redundant compatibility check.

Previously, the closed type family compatibility check was
done even when type-checking an interface file. But interface
files now store compatibility info, so this check was redundant.
Add test case for #8917
Fix #8917.

FamInstEnv.normaliseTcApp should normalise arguments even when
the top-level tycon isn't a type family. This was a regression
from 7.6 -- not sure when it happened, but it was probably my
fault. Fixed now, in any case.