This reverts commit e7d2a62c due to a
bug.

The package databases obtained from GHC_PACKAGE_PATH should be stored in
the LocalBuildInfo and passed to GHC when necessary; otherwise, the user
could change GHC_PACKAGE_PATH causing GHC and Cabal to see inconsistent
installed package sets. cabal-install should trigger reconfiguration if
GHC_PACKAGE_PATH changes.