[wpilib] Remove MotorController::StopMotor() (#8483)

It does the same thing as Disable() in practice, and MotorSafety has its
own StopMotor() function.

wpilibc/src/test/native/include/motorcontrol/MockMotorController.hpp

@@ -18,7 +18,6 @@ class MockMotorController : public MotorController {
   void SetInverted(bool isInverted) override;
   bool GetInverted() const override;
   void Disable() override;
-  void StopMotor() override;
 
  private:
   double m_speed = 0.0;