Clean up EditorManagerPlaceHolder

The fix in acca83add48df wasn't enough: One has to make sure that
even ~EditorManagerPlaceholder doesn't delete EditorManager implicitly,
since it's explicitly deleted in ~MainWindow.

Anyway, EditorManagerPlaceHolder::current() is never used, so we
can get rid of this static variable entirely and simplify the code
by just checking for EditorManager::parent() in the PlaceHolder
destructor.

Change-Id: I3a0374d20c9f3f9d2680cf8302953a336670fc8a
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>