[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.

wpilibjIntegrationTests/src/main/java/edu/wpi/first/wpilibj/MockSpeedController.java

@@ -37,9 +37,4 @@ public class MockSpeedController implements SpeedController {
   public void stopMotor() {
     disable();
   }
-
-  @Override
-  public void pidWrite(double output) {
-    set(output);
-  }
 }