[wpilib] Remove deprecated Controller class (#4440)

Now that old command-based has been removed, this base class can be
removed too.

wpilibc/src/main/native/include/frc/Controller.h

@@ -1,41 +0,0 @@
-// 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 <wpi/deprecated.h>
-
-namespace frc {
-
-/**
- * Interface for Controllers.
- *
- * Common interface for controllers. Controllers run control loops, the most
- * common are PID controllers and their variants, but this includes anything
- * that is controlling an actuator in a separate thread.
- *
- * @deprecated Only used by the deprecated PIDController
- */
-class Controller {
- public:
-  WPI_DEPRECATED("Only used by the deprecated PIDController")
-  Controller() = default;
-  virtual ~Controller() = default;
-
-  Controller(Controller&&) = default;
-  Controller& operator=(Controller&&) = default;
-
-  /**
-   * Allows the control loop to run
-   */
-  virtual void Enable() = 0;
-
-  /**
-   * Stops the control loop from running until explicitly re-enabled by calling
-   * enable()
-   */
-  virtual void Disable() = 0;
-};
-
-}  // namespace frc