Oversight in rBb0741e1dcbc5: Outliner: Use right click target element for context menu.

This was guarded by an assert in get_target_element, but it can be
valid to have these assigned to a shortcut (and then perform the action
without an active outliner element).

Now remove the assert and let the operator polls check if we really have
a target element.

note: this basically makes get_target_element obsolete, could call
outliner_find_element_with_flag instead in a ll cases.