Clean up dependency of functor cache on GaudiConfig2

The change introduced with !2537 (merged) (+ the fixed typo in !2670 (merged)) would have never worked since the MergedConfDB2 target is only added after subdirectories (and the if(TARGET MergedConfDB2) in loki_functors_cache()) are processed. In addition, it was incorrectly passed to gaudirun.py.

Here this change is effectively reverted (together with a minor code cleanup without any visible effect). The reason being that the MergedConfDB2 target isn't defined at all for some projects such as Moore. Instead of adding the dependency here, this is left to the caller of loki_functors_cache in each project (e.g. Rec!2163 (merged)).

Related to Rec!2163 (merged)