Remove redundant import; fix note

* Remove the redundant import of `Data.Maybe` from `GHC.Foreign`.

* Fix the note in `GHC.Stack.Types` to give a correct explanation
of the problematic cycle.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3722