Lumber production code fix (!408)

Description

Just a fix for the way the lumber mill production nerf was handled. Uses the set value in gamedata.ini instead of using a new attribute modifier

Changelog

Checklist

I have:

• Tagged my change with the ;,; comment block.
• Ensured that this merge request changes exactly one thing.
• Attached the proper labels to this merge request.
• Added the Merge Request number in the title. e.g. (!42)
• Tested my code.