[commands] Remove deprecated classes and functions (#5409)

Removes:
- PerpetualCommand
- Command.perpetually()
- CommandGroupBase
- Command.IsGrouped() (C++ only)
- Command.SetGrouped() (C++ only)
- Command.withInterrupt()
- ProxyScheduleCommand
- Button
- InternalButton, JoystickButton, NetworkButton and POVButton now subclass Trigger
- Old style Trigger functions:
    - Trigger.whenActive
    - Trigger.whileActiveOnce
    - Trigger.whileActiveContinuous
    - Trigger.whenInactive
    - Trigger.toggleWhenActive
    - Trigger.cancelWhenActive
- CommandScheduler.clearButtons()
- CommandScheduler.addButtons() (Java only)
- Command supplier constructor of SelectCommand

wpilibNewCommands/src/main/native/include/frc2/command/button/POVButton.h

@@ -5,9 +5,8 @@
 #pragma once
 
 #include <frc/GenericHID.h>
-#include <wpi/deprecated.h>
 
-#include "Button.h"
+#include "Trigger.h"
 
 namespace frc2 {
 /**
@@ -18,7 +17,7 @@ namespace frc2 {
  *
  * @see Trigger
  */
-class POVButton : public Button {
+class POVButton : public Trigger {
  public:
   /**
    * Creates a POVButton that commands can be bound to.
@@ -27,11 +26,9 @@ class POVButton : public Button {
    * @param angle The angle of the POV corresponding to a button press.
    * @param povNumber The number of the POV on the joystick.
    */
-  WPI_IGNORE_DEPRECATED
   POVButton(frc::GenericHID* joystick, int angle, int povNumber = 0)
-      : Button([joystick, angle, povNumber] {
+      : Trigger([joystick, angle, povNumber] {
           return joystick->GetPOV(povNumber) == angle;
         }) {}
-  WPI_UNIGNORE_DEPRECATED
 };
 }  // namespace frc2