efi/arm: Drop unnecessary references to efi.systab

Instead of populating efi.systab very early during efi_init() with
a mapping that is released again before the function exits, use a
local variable here. Now that we use efi.runtime to access the runtime
services table, this removes the only reference efi.systab, so there is
no need to populate it anymore, or discover its virtually remapped
address. So drop the references entirely.

Tested-by: Tony Luck <tony.luck@intel.com> # arch/ia64
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>