certbot-compatibility-test: fix breaking tests (#9955)

Recently our test environments were upgraded to use Docker 26, which
enabled ipv6 loopback by default in containers. This caused tests to
start failing due to an nginx test config which was the sole listener
for ipv6.

This simply removes that ipv6 listen directive in the config, and the
archived version we use for testing.

certbot-compatibility-test/nginx/nginx-roundtrip-testdata/79-configs/site-15291

@@ -19,7 +19,6 @@
 
 server {
 	listen 80 default_server;
-	listen [::]:80 default_server ipv6only=on;
 
 	root /usr/share/nginx/html;
 	index index.html index.htm;