hw/net/vmxnet3: Fix leftover unregister_savevm

Commit 78dd48df reworked vmxnet3's live migration but left a straggling
unregister_savevm call.  Remove it, although it doesn't seem to have
any bad effect.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190822111218.12079-1-dgilbert@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry.fleytman@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>