Fix dangling container

If the loadingControl was separate from the zoomControl, removing it
left behind its container. This fixes the issue by calling the
overridden method when the indicator is separate.
Merge branch 'super-remove' of git://github.com/codefox421/Leaflet.loading into codefox421-super-remove
Take `this.zoomControl` into account

Rather than relying on the map's zoomControl, which is not always set, use
`this.zoomControl`
An example with separate === true