[simple theme] activate - SearXNG JavaScript Style Guide

Add the 'SearXNG JavaScript Style Guide' as one quality gate in the build chain
of the simple theme::

    make themes.simple
    make themes.simple.test

In the github CI the `themes.all` target enforce a `themes.simple`.

BTW: Remove 'jshint' left overs from 0ee316f3

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>