[fix] Gigablast.com has been erased

[1] https://www.reddit.com/r/searchengines/comments/128wdcp/gigablastcom_has_been_erased/

Closes: https://github.com/searxng/searxng/issues/2321


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