Remove unused "Encoding": "UTF-8" from plugins' metadata JSON
Summary:
Had slipped in from conversion of metadata in desktop file format,
but are of no use in the JSON variant.
Test Plan: Plugins load and work as before
Reviewers: KDE Connect, nicolasfella
Reviewed By: KDE Connect, nicolasfella
Differential Revision: https://phabricator.kde.org/D11417