Fixes memory leak in KUrlComboBox::setUrl

Summary:
Subject: Fixes memory leak in KUrlComboBox::setUrl

Details:

If setUrl is called multiple times it first removes the previous item
but fails to delete the item before removing it causing a memory leak.

Test Plan: Unit test

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: dfaure, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14666