Filter: remove filter with double type

We are currently not using LowPassFilter2p<double> and it just generates
a lot of warnings on PX4 while instantiating it due to implicitly
promoting float to double:

libraries/Filter/LowPassFilter2p.cpp: In instantiation of
	'T DigitalBiquadFilter<T>::apply(const T&, const DigitalBiquadFilter<T>::biquad_params&) [with T = double]':
libraries/Filter/LowPassFilter2p.cpp:86:41:   required from 'T LowPassFilter2p<T>::apply(const T&) [with T = double]'
libraries/Filter/LowPassFilter2p.cpp:98:16:   required from here
libraries/Filter/LowPassFilter2p.cpp:20:82: warning: implicit conversion from 'float' to 'double' to match other
	operand of binary expression [-Wdouble-promotion]
     T delay_element_0 = sample - _delay_element_1 * params.a1 - _delay_element_2 * params.a2;
                                                                                  ^

libraries/Filter/LowPassFilter2p.cpp

@@ -93,6 +93,5 @@ T LowPassFilter2p<T>::apply(const T &sample) {
 template class LowPassFilter2p<int>;
 template class LowPassFilter2p<long>;
 template class LowPassFilter2p<float>;
-template class LowPassFilter2p<double>;
 template class LowPassFilter2p<Vector2f>;
 template class LowPassFilter2p<Vector3f>;

libraries/Filter/LowPassFilter2p.h

@@ -88,9 +88,8 @@ LowPassFilter2p<T>::LowPassFilter2p(float sample_freq, float cutoff_freq) {
 typedef LowPassFilter2p<int>      LowPassFilter2pInt;
 typedef LowPassFilter2p<long>     LowPassFilter2pLong;
 typedef LowPassFilter2p<float>    LowPassFilter2pFloat;
-typedef LowPassFilter2p<double>   LowPassFilter2pDouble;
 typedef LowPassFilter2p<Vector2f> LowPassFilter2pVector2f;
 typedef LowPassFilter2p<Vector3f> LowPassFilter2pVector3f;
 
 
-#endif // LOWPASSFILTER2P_H
+#endif // LOWPASSFILTER2P_H