[wpilib, examples] Cleanup PotentiometerPID, Ultrasonic, UltrasonicPID examples (#4893)

Fix C++ Ultrasonic to return correct units.

wpilibcExamples/src/main/cpp/examples/Ultrasonic/include/Robot.h

@@ -0,0 +1,25 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <frc/TimedRobot.h>
+#include <frc/Ultrasonic.h>
+
+/**
+ * This is a sample program demonstrating how to read from a ping-response
+ * ultrasonic sensor with the {@link Ultrasonic class}.
+ */
+class Robot : public frc::TimedRobot {
+ public:
+  void RobotInit() override;
+  void TeleopPeriodic() override;
+  void TestInit() override;
+  void TestPeriodic() override;
+  void TestExit() override;
+
+ private:
+  // Creates a ping-response Ultrasonic object on DIO 1 and 2.
+  frc::Ultrasonic m_rangeFinder{1, 2};
+};