board/pc: drop unused grub-efi.cfg file

Following commit fee29b05
("configs/pc_x86_64_efi: use a GPT partition table"),
board/pc/grub-efi.cfg is no longer used anywhere: the
post-image-efi-gpt.sh script generates the grub configuration.

Also, since post-image-efi-gpt.sh generates a grub configuration file
that uses the root filesystem partition UUID as the root= kernel
argument, the instructions in the readme.txt file to tweak root= from
/dev/sda2 to /dev/vda2 is no longer relevant. This was noted in the
commit log of fee29b05:

    The root filesystem location is passed to the kernel by a partition
    UUID, so it is possible to boot on QEMU, directly from the disk image,
    or dump the image to a physical device.

Fixes: #11841
Cc: Pete Morici <pmorici@dev295.com>
Cc: Carlos A. M. dos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>