[fix] simple theme: remove static files no longer needed

remove:

    searx/static/themes/simple/js/searxng.js
    searx/static/themes/simple/js/searxng.head.js
    searx/static/themes/simple/css/searxng-rtl.css
    searx/static/themes/simple/css/searxng.css

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