xen/arm: Drop support for loading ELF Dom0 kernel

xen/arm: Drop support for loading ELF Dom0 kernel

The code has been around since the beginning of Xen Arm. However, I am
not aware of any user and the code is pretty bogus:
    1) It is assuming virtual address == physical address.
    2) The cache is not cleaned after the Image is loaded but the Image
    is started with Cache disabled.
    3) There are not clear ABI with the guest.

Xen is currently supporting 3 other formats (zImage, Image, U-boot Image)
as well as gzip compressed version of each formats. All of them are well
documented and widely use.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>