Remove installer.grub1 from the release-critical builds

It randomly crashes

  Kernel panic - not syncing: assertion "i && sym_get_cam_status(cp->cmd) == DID_SOFT_ERROR" failed: file "/tmp/nix-build-linux-3.14.43.drv-0/linux-3.14.43/drivers/scsi/sym53c8xx_2/sym_hipd.c", line 3399

Rumour has it this is fixed in newer versions of QEMU or the kernel,
so I'm only disabling it on the 14.12 branch.

http://hydra.nixos.org/build/22836392

nixos/release-small.nix

@@ -43,7 +43,6 @@ in rec {
         simple;
       installer = {
         inherit (nixos'.tests.installer)
-          grub1
           lvm
           separateBoot
           simple;