xen/arm: Remove the makefile target xen.axf

xen/arm: Remove the makefile target xen.axf

Since commit 4557c22 "xen: arm: rewrite start of day page table and cpu
bring up", Xen requires to be launched in NS HYP/EL2.

xen.axf is generated in order to directly boot Xen on ARM models (e.g
Foundation). However they usually start in secure mode, which mean Xen
cannot boot.

The way forward to boot Xen on models is using either EFI or
bootwrapper [1].

[1] https://git.kernel.org/cgit/linux/kernel/git/mark/boot-wrapper-aarch64.git/

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