Remove documentation of Flite license as part of Qt Speech.

Qt Speech uses Flite as a 3rd-party dependency. The Flite copyright and license is documented in https://doc.qt.io/qt-5/legal-flite.html, that also shows up in the general License page: https://doc.qt.io/qt-5/licenses-used-in-qt.html#additional-information .

Anyhow, Qt Speech source code does not contain any Flite code, just enablers to link against the library. Therefore the documentation is misleading.