[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/cpp/frc2/command/CommandScheduler.cpp

@@ -107,10 +107,6 @@ frc::EventLoop* CommandScheduler::GetDefaultButtonLoop() const {
   return &(m_impl->defaultButtonLoop);
 }
 
-void CommandScheduler::ClearButtons() {
-  m_impl->activeButtonLoop->Clear();
-}
-
 void CommandScheduler::Schedule(Command* command) {
   if (m_impl->inRunLoop) {
     m_impl->toSchedule.emplace_back(command);