Portage/GHCCore.hs: drop coreLibs logic

It masks our long-standing resolver bug:

When only test dependencies require upper ghc version
via dependencies like deepseq we leave deepseq depend
itself in generated ebuild, while we should change it
to >=ghc-7.4.1.

Observed in lens-4.1 ebuild.

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