remove own implementation of QString to TString conversion for taglibwriter

Summary:
as pointed out in D13700, the custom function for conversion of taglib string
to QString is unsafe and will break if size(wchar) == 4 bytes. Use the function
provided by taglib for the conversion to TString. See also D15614

Test Plan: tests pass

Reviewers: mgallien, bruns, svuorela

Reviewed By: bruns

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15715