Remove unused entry X-KDE-DBus-ModuleName from kded plugins' metadata

Summary:
That entry has never been used, instead the plugin id (as by
KPluginMetaData::pluginId()) has been used for the module name in the
D-Bus object path.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13622