Remove grub_efi_allocate_pages.

Remove grub_efi_allocate_pages.

grub_efi_allocate_pages Essentially does 2 unrelated things:
* Allocate at fixed address.
* Allocate at any address.

To switch between 2 different functions it uses address == 0 as magic
value which is wrong as 0 is a perfectly valid fixed adress to allocate at.