arm64: Remove unused typedef

We don't use EFI_MEMORY_DESCRIPTOR that's typedef'd here. We use the one
from sys/efi.h instead. Remove the clutter here as these two are subtly
different (though wind up with the same layout due to alignment rules).

Sponsored by: Netflix

(cherry picked from commit 211053f952d8ac91dab239f6c4845dd5badd0d95)