Remove some unused SceneTree properties

[berarma]

This is merely clean up work. While reading the code I've found some SceneTree properties doing nothing really. Probably remains from previous versions.

[akien-mga]

Good catch, thanks.

I checked the history and:

• root_lock was never used since Godot was open sourced
• tree_version stopped being used in d61b91c
• processing was added in 98c655e but then not used in the end

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[berarma]

Thanks! 😃