Previously, checkTauTvUpdate, used in the eager unifier (TcUnify)
right before writing to a metavar, refused to write a metavar to
a type involving type functions. The reason for this was given
in a Note, but the Note didn't make all that much sense and even
admitted that it was a bit confused. The Note, in turn, referred
to another Note, which it was quite sceptical of, as well.

The type-family check was slowing down performance, so I tried
removing it, running the tests referred to in the Note. The tests
all passed without the check. Looking at more test results, I
saw several error messages improve without the check, and some cases
where GHC looped (T7788, in particular) it now doesn't.

So, all in all, quite a win: Two hairy Notes removed, several lines
of code removed, better performance, and improved output.

[skip ci]