MIPS: KVM: remove the stale memory alias support function unalias_gfn

The memory alias support has been removed since a1f4d395 (KVM: Remove
memory alias support). So remove unalias_gfn from the MIPS port.

Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>