Map overlays for mainline campaigns

Delete the unused localized map overlays for the old style maps, leaving only the titlescreen (as it is in core, UMC might use it). This saves about 5 MB on the mainline campaigns.

I'll add a full set of German overlays for the new bigmaps as soon as an issue with the script is fixed. If the new release is tagged too early for that, I'll make a separate PR for them.