[lookandfeel kcm] Do not declare plugin in lookandfeeltool code version

Summary:
Also improves race condition on creating the JSON file and running
automoc over the cpp file which refers to it, given there is no
dependency chain defined at all for the lookandfeeltool target and
that kcm_lookandfeel.json, which was prone to make highly parallel
builds fail

Reviewers: #freebsd, tcberner, bshah, mart, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, rikmills, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10485