[wpilib] Remove deprecated Controller class (#4440)

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

wpilibj/src/main/java/edu/wpi/first/wpilibj/Controller.java

@@ -1,23 +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.
-
-package edu.wpi.first.wpilibj;
-
-/**
- * An interface for controllers. Controllers run control loops, the most command are PID controllers
- * and there variants, but this includes anything that is controlling an actuator in a separate
- * thread.
- *
- * @deprecated None of the 2020 FRC controllers use this.
- */
-@Deprecated(since = "2020", forRemoval = true)
-public interface Controller {
-  /** Allows the control loop to run. */
-  void enable();
-
-  /**
-   * Stops the control loop from running until explicitly re-enabled by calling {@link #enable()}.
-   */
-  void disable();
-}