When the tab label gets removed from the notebook on widget
desctruction, we should still unconditionally unparent it from the
notebook, since failing to do so will keep a stray reference alive.

In case applications rely on the tab label being destroyed to release
other references (e.g. because the tab label is a custom object, or
another object's lifecycle is tied to it using g_object_set_data_full()),
this will also possibly cause other references to get leaked.
In Nautilus, the result was we were failing to release the reference to
a NautilusWindowSlot, and other parts of the application relied
on it being destroyed at a specific time instead, causing the
application to crash when closing a window.

This is a regression from commit
325cf071.

This commit restores the previous unparenting behavior in case we're not
in a DnD operation.

https://bugzilla.gnome.org/show_bug.cgi?id=680349