Clean up Mozilla specific code (#11120)
* Don't fetch NODE_CHILD_OF when fetching parent, since this is no longer needed (or useful) for combo boxes.
* Get rid of mozilla.RootApplication, since this object is no longer reachable without NODE_CHILD_OF.
* accParent is broken in Firefox for same-process iframe documents, so we must still use NODE_CHILD_OF for Documents (but nowhere else).
* Gecko now prunes aria-hidden objects from the tree, so the hidden attribute is no longer used.
* NVDA core: Drop code specific to supporting very old versions of Gecko.
* Gecko vbuf backend: Drop code specific to supporting very old versions of Gecko.
* Gecko always sets the focused state correctly now, so drop BrokenFocusedState code.
* Remove unnecessary imports.
* We only allow show/hide/reorder events for specific cases anyway, so the code to ignore these events for the Firefox location bar is redundant.
* Firefox hasn't had a status bar for a long time, so the Firefox app module does nothing. Remove it.
