arm64: cleanup unused UDBG_* define

The UDBG_UNDEFINED/SYSCALL/BADABORT/SEGV are only used to show
verbose user fault messages in arm, not arm64, drop them.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>