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

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

xrpVendordep/src/main/java/org/wpilib/xrp/XRPMotor.java

@@ -103,9 +103,4 @@ public class XRPMotor implements MotorController {
   public void disable() {
     set(0.0);
   }
-
-  @Override
-  public void stopMotor() {
-    set(0.0);
-  }
 }