Removed the option to disable the entire cache

Originally intended for developers, this option has caused a lot of troubles without any real use. Turning it off can cripple an entire system, e.g. the entire installation system will blow up when the cache is disabled.

In fact it has no real use, if you want to debug a cache builder, you could just add an print_r() and exit() in getData() and then clear the cache. Profit.
Merge branch 'master' of github.com:WoltLab/WCF