Move the operator selection to the Opt4J framework

* The operator selection is now embedded in the Opt4J framework, so
  remove all code in this plugin related to that feature.
* The previous implementation was buggy such that multiple operators
  could be bound to one genotype, but only one of them was executed per
  exploration run (random selection).
* Fixing these issues in this plugin would have been a suboptimal
  solution since the required selection logic would need to be
  duplicated in each module binding multiple operators.

Signed-off-by: Alexander Diewald <diewald@fortiss.org>