Fixed "unknown command" when a non-existent setting is found in the DB

If a FPP.Setting is removed then it will still exist in the DB. When these settings are retrieved in the DB then FPP an "unknown command" message will be printed to the console twice every time a player joins the server.

This commit checks if a setting exists and if not, removes it from the DB and skips to the next setting.

Not much of a big deal at the moment but this future proofs things if a setting were to be removed. Or maybe if someone has an old FPP database then this'll clean it up for them.

Also, how have you got merging into darkrpFPP set up? Should I submit another PR for that since it uses MySQLite rather than FPPDB?