Revert "Try harder to pick the correct version in cabalLibVersionToUse."

This reverts commit a17491d70741d28d9b6037640e6fb2b041fc81bc.

That commit made the logic much more convoluted and broke the following use case
(for sandboxed packages with build-type Custom):

$ cabal configure -w /path/to/ghc
$ cabal build
Merge remote-tracking branch 'upstream/master'