Oscillator: reverted recent fix for PM

As of acaaea8204 a samplerate dependent
ratio has been added into calculation of the local phase. This however is
wrong for PM.
(cherry picked from commit 2547bf8d57)

src/core/oscillator.cpp

@@ -375,13 +375,11 @@ void oscillator::updatePM( sampleFrame * _ab, const fpp_t _frames,
 	m_subOsc->update( _ab, _frames, _chnl );
 	recalcPhase();
 	const float osc_coeff = m_freq * m_detuning;
-	const float sampleRateCorrection = 44100.0f /
-				engine::getMixer()->processingSampleRate();
 
 	for( fpp_t frame = 0; frame < _frames; ++frame )
 	{
 		_ab[frame][_chnl] = getSample<W>( m_phase +
-					_ab[frame][_chnl]*sampleRateCorrection )
+					_ab[frame][_chnl] )
 							* m_volume;
 		m_phase += osc_coeff;
 	}