Revert "Merge pull request #2449 from wkennington/master.grub"

This reverts commit 469f22d717e53c48d13a66ca862942e8098accc5, reversing
changes made to 0078bc5d8f87512104902eab00c8a44bef286067.

Conflicts:
	nixos/modules/installer/tools/nixos-generate-config.pl
	nixos/modules/system/boot/loader/grub/install-grub.pl
	nixos/release.nix
	nixos/tests/installer.nix

I tried to keep apparently-safe code in conflicts.