KVM: ARM: Remove kvm_arch_set_memory_region()

This was replaced with prepare/commit long before:

  commit f7784b8e
  KVM: split kvm_arch_set_memory_region into prepare and commit

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>