[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/cpp/motorcontrol/MockMotorController.cpp

@@ -25,7 +25,3 @@ bool MockMotorController::GetInverted() const {
 void MockMotorController::Disable() {
   m_speed = 0;
 }
-
-void MockMotorController::StopMotor() {
-  Disable();
-}