Remove generic bird egg, replace its role with chicken egg

[Karol1223]

Summary

None

Purpose of change

Our generic wild bird egg has actually been unused for anything but a glorified template for a while now. All values it had were based off chicken eggs anyway, so we can just cut out the middleman and use the chicken egg as the template (which, in vanilla, we already were doing)

Describe the solution

As the item was not actually present ingame anywhere save for the crafting UI, this gives no gameplay changes without mods

With mods, however, MoM players will now have a properly updated bird egg requirement group. The JSON comment was lying - it is perfectly possible to extend requirement groups without errors - as I did, and double tested to be sure

Describe alternatives you've considered

Testing

Additional context

No item migration because the item wasn't available outside debug anyway. If people more important than me wish for me to handle the migration regardless of this, I will.

This is going to require tileset files to be renamed after its merged.