efi: libstub: drop pointless get_memory_map() call

Currently, the non-x86 stub code calls get_memory_map() redundantly,
given that the data it returns is never used anywhere. So drop the call.

Cc: <stable@vger.kernel.org> # v4.14+
Fixes: 24d7c494 ("efi/arm-stub: Round up FDT allocation to mapping size")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>