Remove the FacetapiDependencyFacet::getActiveFacets() method in favor of the FacetapiDependency::activeItems property

The FacetapiDependencyFacet::getActiveFacets() method parses the query string to get the active facet items. The problem with this approach is that it depends on the $_GET['f'] query string variable, which may or may not be where facet data is stored depending on which URL processor plugin is being used by the searcher. In addition, the FacetapiDependency parent class actually provides this information already via the $this->activeItems property. Therefore we can reduce code and eliminate hard coding $_GET['f'] in the application logic.