Don't load KDE platform module in xembedsniproxy

Summary:
xembedsniproxy doesn't do anything that requires it to be
aware of KDE GUI settings (colors, style, standard keys, etc).

This makes it start a fair bit faster.

Reviewers: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9194