Remove unnecessary imports in GHC.Event.KQueue to fix compiler warnings.

Summary: The imports of Data.Monoid and Data.Maybe in GHC.Event.KQueue are unnecessary and cause validate to fail.

Test Plan: Validate successfully (though I get some unrelated failures).

Reviewers: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D217