Remove workaround for ownership issue with newly created tabs

The workaround for specifying the loader as a parent for newly
created tabs can be removed, now that QTBUG-35913 is fixed and
the visual parent ensures that its visual children stay alive.

Change-Id: Ide3fc79c7a7465bd585bfd2bc66de4389aa7bc1d
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>