This is annoying because it's not possible to use StartUndo/EndUndo
because that would break grouping via SwUndoInsert::CanGrouping();
also SwUndoAttr is somehow incapable of removing the inserted hints of a
grouped insert (it seems to leave no-length hints behind); so add an
explicit call to DeleteAttributes which should avoid the no-length
hints.

Change-Id: I1533daed9b2cf59886f380141b4eace4b22c15e0