[fix] about page- 'How to set as the default search engine?'

Searx supports OpenSearch, remove the inline JS from the about page.

close: https://github.com/asciimoo/searx/issues/1949
close: https://github.com/asciimoo/searx/issues/1616



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