Remove HRTFs for Avatar too

Turns out there was already a function for that, if you just call
removeHRTFForStream without a 2nd param, it appears to just delete
the audio with uuid of QUuid() which is the avatar's mic.  That is
what we want, so cool.

assignment-client/src/audio/AudioMixer.cpp

@@ -603,6 +603,9 @@ void AudioMixer::handleKillAvatarPacket(QSharedPointer<ReceivedMessage> packet,
     auto clientData = dynamic_cast<AudioMixerClientData*>(sendingNode->getLinkedData());
     if (clientData) {
         clientData->removeAgentAvatarAudioStream();
+        // when you don't specify a stream, this removes just the QUuid() 
+        // stream (which is the avatar's mic stream)
+        clientData->removeHRTFForStream(sendingNode->getUUID());
     }
 }