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

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

wpilibj/src/test/java/org/wpilib/hardware/motor/MockMotorController.java

@@ -33,9 +33,4 @@ public class MockMotorController implements MotorController {
   public void disable() {
     m_speed = 0;
   }
-
-  @Override
-  public void stopMotor() {
-    disable();
-  }
 }