replace file system check with WF check

`WikiFactory::getVarIdByName()` calls `WikiFactory::loadVariableFromDB()` and the only "caching" is to avoid fetching the same data more than once during a single HTTP request. Many `WikiFactory` methods rely on `WikiFactory::loadVariableFromDB()` and I don't feel like messing with it at this particular moment

It is generally "safe" to not cache the value in memcached - the database will be queried once per wiki creation process, ca. 600 times per day. Given the fact, that the wiki creation process takes times, one additional query won't hurt us. I agree caching here is overkill and impairs readability.