Cleanup of CutFlowSvc properties and deprecated methods

This is an initial cleanup of the CutFlowSvc.

• Removing addEvent without passing the event weight as it is deprecated.
• Moving variable initialisations into the header.
• Removing OutputIncompleteCollName and SkimmingCycle properties as they are not used.
• No need for an event store instance.
• Cleaning-up empty overloaded methods.
• Removing an unused client from DerivationFrameworkCore.
• Some tests have also been cleaned up, but more work is needed on this front.

/cc @tsulaia