Removed useless spectrum visualizer in NFM receivers. Created a null sink class to fit corresponding parameter in NFMDemod class constructor

sdrbase/dsp/nullsink.cpp

@@ -0,0 +1,43 @@
+#include "dsp/nullsink.h"
+#include "dsp/dspcommands.h"
+#include "util/messagequeue.h"
+
+NullSink::NullSink()
+{
+}
+
+void NullSink::feed(SampleVector::const_iterator begin, SampleVector::const_iterator end, bool positiveOnly)
+{
+}
+
+void NullSink::start()
+{
+}
+
+void NullSink::stop()
+{
+}
+
+bool NullSink::handleMessage(Message* message)
+{
+	message->completed();
+	return true;
+	/*
+	if(DSPSignalNotification::match(message)) {
+		DSPSignalNotification* signal = (DSPSignalNotification*)message;
+		m_sampleRate = signal->getSampleRate();
+		message->completed();
+		return true;
+	} else if(DSPConfigureScopeVis::match(message)) {
+		DSPConfigureScopeVis* conf = (DSPConfigureScopeVis*)message;
+		m_triggerState = Untriggered;
+		m_triggerChannel = (TriggerChannel)conf->getTriggerChannel();
+		m_triggerLevelHigh = conf->getTriggerLevelHigh() * 32767;
+		m_triggerLevelLow = conf->getTriggerLevelLow() * 32767;
+		message->completed();
+		return true;
+	} else {
+		return false;
+	}
+	*/
+}