Remove cabal.config from Haskell.

The Stackage project provides a cabal.config which locks in a specific
package set.  This file should be shared between developers project so
that everyone is developing using the same versions of packages.

Haskell.gitignore

@@ -11,7 +11,6 @@ cabal-dev
 .hsenv
 .cabal-sandbox/
 cabal.sandbox.config
-cabal.config
 *.prof
 *.aux
 *.hp