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: #11371