haskell-cabal.eclass: stop unsetting LC_ALL, LANG, LC_MESSAGES

It either should be forced in all phases ('src_configure()' / 'src_install()')
or in none.

LANG=C is a bit too harsh for modern environment.
Should be something like 'LC_CTYPE=UTF-8' instead.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>