Remove overmap_debug_mongroup

[KheirFerrum]

Summary

SUMMARY: Bugfixes "Remove overmap_debug_mongroup"

Purpose of change

While porting #2089 I accidentally included this into savegame_json.cpp under write but not read. It's a variable used in DDA but not in BN that I accidentally included while solving merge conflicts.

Describe the solution

Removes overmap_debug_mongroup from savegame_json.cpp entirely.

Describe alternatives you've considered

• Add in whatever it's supposed to reference.
  I actually have no clue what it actually does and looking for it might take a while. I'd rather do that some other time, especially if it turns out that it's not something we even want.

Testing

Save load, then turn on "Report Unused JSON Fields", verified that the error no longer triggered.

Additional context

I really need to set "Report Unused JSON Fields" as true for all my test builds.