The simplest but most manual labour intensive way is to go through it and comment out everything related to non god content, then in history/characters/mythos_characters find every character that requires those commented out things and modify them to regular people (remove vampire trait, change draconic culture and religion to norse or finnish) repeat with titles. Problems might arise with the checked_race flag as I am not sure if that interacts with god content.
Technically you could save a lot of time by just deleting code instead of commenting out but then it might be more difficult to hunt down bugs.
And yes it is a lot of work.
Another way is to only remove the stuff that spawns mythos races (and traits, like monster hunter) then remove all examples of them (in game or in history files) and maybe also remove the magic system via the comment out method.