[DocumentUrlDB] Remove unused 'rename' method

Summary: It is not used anywhere, so remove it.

Test Plan: still compiles

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: shubham, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21692