Don't recompute some free vars in lintCoercion
As pointed out by @simonpj on D2044 we don't need
to compute the free vars of the range of the substitution
as most of them are already carried by the monad.
This should be a tiny performance improvement over the version
from before D2044.
Also removes an extra function that is now unnecessary.
Test Plan: ./validate && ./validate --slow
Reviewers: goldfire, simonpj, austin, bgamari
Reviewed By: simonpj
Subscribers: thomie, simonmar, simonpj
Differential Revision: https://phabricator.haskell.org/D2060
GHC Trac Issues: Trac #11371