Avoid serializing base64 encoded favicon data twice

Summary:
The URL is ignored in case the image data is available. Avoid pushing
it twice via DBus.

Test Plan:

1. start dbus-monitor
2. type some string in krunner

• before: dict entries contain both favIconData and favIconUrl, with identical data
• after: only favIconData is part of the dict

Favicons are still shown in the krunner results

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19963