Remove unreliable Core Lint empty case checks

Trac #13990 shows that the Core Lint checks for empty case are
unreliable, and very hard to make reliable. The consensus (among
simonpj, nomeata, and goldfire) seems to be that they should be
removed altogether. Do that.

Add test

Reviewers: goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13990

Differential Revision: https://phabricator.haskell.org/D4161