[wpilib] Remove PIDController, PIDOutput, PIDSource

Move them to the old commands vendordep so that PIDCommand and PIDSubsystem
continue to work.

This also removes Filter and LinearDigitalFilter.

wpilibc/src/test/native/include/MockSpeedController.h

@@ -17,8 +17,6 @@ class MockSpeedController : public SpeedController {
   void Disable() override;
   void StopMotor() override;
 
-  void PIDWrite(double output) override;
-
  private:
   double m_speed = 0.0;
   bool m_isInverted = false;