Remove Cheats/ExtraCheats distinction

It is not necessary as there is no longer a need to preserve
the size/order of struct Cheats

src/sl/cheat_sl.cpp

@@ -26,10 +26,10 @@ struct ExtraCheatNameDesc {
 };
 
 static ExtraCheatNameDesc _extra_cheat_descs[] = {
-	{ "inflation_cost",   &_extra_cheats.inflation_cost },
-	{ "inflation_income", &_extra_cheats.inflation_income },
+	{ "inflation_cost",   &_cheats.inflation_cost },
+	{ "inflation_income", &_cheats.inflation_income },
 	{ "station_rating",   &_cheats.station_rating },
-	{ "town_rating",      &_extra_cheats.town_rating },
+	{ "town_rating",      &_cheats.town_rating },
 };
 
 static const SaveLoad _cheats_desc[] = {