Drop tests.installer.grub1 as release critical

This test sometimes fails with

  Kernel panic - not syncing: assertion "i && sym_get_cam_status(cp->cmd) == DID_SOFT_ERROR" failed: file "/tmp/nix-build-linux-3.14.32.drv-0/linux-3.14.32/drivers/scsi/sym53c8xx_2/sym_hipd.c", line 3399

after "sd 2:0:0:0: ABORT operation timed-out."

Since we don't care all that much about GRUB 1 anymore, don't make the
release depend on it.

http://hydra.nixos.org/build/19563197