Remove the Callback GC hack for users upgrading from pre-3.0.0
Remove the Callback GC hack for users upgrading from pre-3.0.0

etodd/Steamworks.NET,rlabrecque/Steamworks.NET,GoeGaming/Steamworks.NET,rlabrecque/Steamworks.NET,itamargreen/Steamworks.NET,Yukinii/Async-Await-Steamworks.NET,rlabrecque/Steamworks.NET,Gert-Jan/Steamworks.NET,rlabrecque/Steamworks.NET