Fixes ghci :unset -X<ext> so that it doesn't fail to reverse option. (fixes trac #9293)

Summary: ghci unset could not reverse language extensions.

Reviewers: hvr, thomie, austin

Reviewed By: hvr, thomie, austin

Subscribers: goldfire, hvr, thomie, carter

Differential Revision: https://phabricator.haskell.org/D516

GHC Trac Issues: #9293