GH-40 NextOnTarget Click event not always removed

Remove on click listener with nextOnTargetClick callback for following cases:
  * changing step with nextStep or prevStep call (removeEvtListener in changeStep function)
  * changing step directly via showStep call (removeEvtListener in showStepHelper function)
  * ending tour via closeTour call or clicking on close button (removeEvtListener inside endTour function)

Additionally, removing few unused variables throughout the code.