Fix faction description not being loaded from save.

Fixes #9895

This was changed to "description" by bf8aa0f0892cf4c3458ef0cace3480c6bd855b03, but should not apply to loading saves (the string extractor only considers json files in data/, but not in save/). The faction description is still written to the member "desc" (see `faction::serialize`).

The old descriptions are lost, the game previously loaded from "description" (did not exist, therefor empty string) and upon save, stored that empty string as "desc". It works fine in new worlds.