[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/cpp/MockSpeedController.cpp

@@ -29,7 +29,3 @@ void MockSpeedController::Disable() {
 void MockSpeedController::StopMotor() {
   Disable();
 }
-
-void MockSpeedController::PIDWrite(double output) {
-  Set(output);
-}