Remove unused variable in GDScriptLanguage

[YYF233333]

This line of code has been commented out for at least four years, nobody should need that anymore.

And without that usage, variable calls is obsoleted (despite initialization), remove to save some space.

[dalexeev]

Makes sense to me. I doubt the total number of function calls per frame would be useful for performance debugging. Even if we need it, we can always add it back.

[akien-mga]

Thanks!