Kernel: Remove outdated FIXME in the DeviceManagement code

Author: https://github.com/supercomputer7
Commit: https://github.com/SerenityOS/serenity/commit/6f9b84a64a
Pull-request: https://github.com/SerenityOS/serenity/pull/16991

Kernel/Devices/DeviceManagement.h

@@ -73,7 +73,6 @@ private:
     LockRefPtr<NullDevice> m_null_device;
     LockRefPtr<ConsoleDevice> m_console_device;
     LockRefPtr<DeviceControlDevice> m_device_control_device;
-    // FIXME: Once we have a singleton for managing many sound cards, remove this from here
     SpinlockProtected<HashMap<u64, Device*>, LockRank::None> m_devices {};
 
     mutable Spinlock<LockRank::None> m_event_queue_lock {};