Disable stats module in release mode

This module is only used in debug code and so triggers an unused code warning when building geelightning in release mode (such as when building geewallet on gitlab CI).

This had been disabled before but the #[cfg(debug_assertions)] attribute somehow got moved to the scripts mod declaration, presumably during a rebase, which solves the mystery of why the scripts module was disabled and had to be re-enabled in the previous commit.