Removed code which causes Godot 3 to report memory leaks (#57)

[puzzud]

Godot 3 does not support classes referencing their own class type inside their own functions. Using the is keyword in combination will throw a parser error. But using the as keyword similarly does not. Its apparent downstream affect is at least reports of memory leaks.

I have tagged #57 in an associated Godot repo issue and appealed for a Godot maintainer to target Godot 3.6 as it was targeted to Godot 3.5, proposing a solution to guard against this usage of as, as it's unlikely to receive the circular dependency enhancement that Godot 4 received--this nakama-godot issue does not reproduce with Godot 4.

[CLAassistant]

All committers have signed the CLA.