"facetapi_filter_inactive_parent" is not reflective of what the function actually does

When processing hierarchies, items are copied to their parents as children and then stripped from the top level. In other words, it is a two-step nesting process. The function that strips the children form the top level (i.e. step 2) is currently named facetapi_filter_inactive_parent() which is not reflective of what it actually does. The function should be renamed and documented to it is more transparent as to what it is actually doing.