nic: do not destroy memory regions in cleanup functions

The memory regions should be destroyed in the unrealize function;
since these NICs are not even qdev-ified, they cannot be unplugged
and they do not have to do anything to destroy their memory regions.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>