keep installing metadata.desktop

there are too many existing users that rely on existence of metadata.desktop,
so keep installing it, even if with new plasma-workspace releases only the
metadata.json would be used. the framework should never change its behavior
wrt old software, even if it was doing something in a sense wrong
CCMAIL:	Aleix Pol Gonzalez <aleixpol@kde.org>
BUG:372594