x86: remove arch specific early_init_dt_alloc_memory_arch

Now that the DT core code handles bootmem arches, we can remove the x86
specific early_init_dt_alloc_memory_arch function.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>