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

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

wpilibc/src/main/native/include/wpi/hardware/motor/MotorController.hpp

@@ -61,11 +61,6 @@ class MotorController {
    * Common interface for disabling a motor.
    */
   virtual void Disable() = 0;
-
-  /**
-   * Common interface to stop the motor until Set is called again.
-   */
-  virtual void StopMotor() = 0;
 };
 
 }  // namespace wpi