memory: Eliminate memory_region_destructor_ram_from_ptr()

The function is equivalent to memory_region_destructor_ram(), so
it's not needed anymore.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1446844805-14492-3-git-send-email-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>