drop obsolete 40grub overlay symlink

40grub was dropped from f-c-c in [1] since we are now
using the `ignition-grub` package to provide those bits.
Let's drop it here too.

[1] coreos/fedora-coreos-config@74cd237

Author: dustymabe

common.yaml

@@ -24,7 +24,6 @@ ostree-layers:
   - overlay/30rhcos-nvme-compat-udev
   - overlay/30gcp-udev-rules
   - overlay/30lvmdevices
-  - overlay/40grub
 
 conditional-include:
   - if: basearch != "s390x"