LibCore: Completely drop IODevice

Finally, the end of our journey to fallible I/O!

Now all that's left to do is to rewrite the code around each and every
.release_value_but_fixme_should_propagate_errors() call. :^)

Closes #17129.

Userland/Libraries/LibCore/Forward.h

@@ -21,7 +21,6 @@ class ElapsedTimer;
 class Event;
 class EventLoop;
 class File;
-class IODevice;
 class LocalServer;
 class LocalSocket;
 class MappedFile;