build-grub: remove all the code unrelated to grub build

Remove all the code related to preparation of the disk image
for busybox and OE file systems from build-grub.sh file. This
file should contain code that is only related build/package
stage of grub.

Change-Id: Id9fcccd560c03d6f9d67b8887f370398b450a1ff
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>