csharp: FunctionPtr wrappers are not IDisposable
They don't actually store unmanaged memory and we already rely on the GC
to free them (not calling Dispose() directly through the managedCb
closure, so we don't need IDisposable.
Just making sure we release the Function Pointer data in the main loop
as the GC runs in a different thread.
Depends on D10672
Reviewers: brunobelo, felipealmeida, YOhoho
Reviewed By: brunobelo
Maniphest Tasks: T8423
Differential Revision: https://phab.enlightenment.org/D10673