nginx: move the fastcgi cache configuration to the archwiki role

This is much cleaner because the nginx role does not have to set the
fastcgi_cache variable to "false" by default, which was overridden by
host_vars/apollo.archlinux.org to "wiki", but the value was still
hardcoded in the config.

At first, I was wondering that the cache "zone" name should be
generalized to improve the configuration (from the original per-host to
per-service), but that would be an overkill since the fastcgi cache is
used only for the wiki...

host_vars/apollo.archlinux.org

@@ -36,5 +36,3 @@ fail2ban_jails:
   sshd: true
   postfix: true
   dovecot: false
-
-fastcgi_cache: wiki

roles/archwiki/templates/nginx.d.conf.j2

+fastcgi_cache_path /etc/nginx/wikicache levels=1:2 keys_zone=wiki:100m inactive=60m;
+fastcgi_cache_key "$scheme$request_method$host$request_uri";
+
 upstream archwiki {
     server unix://{{ archwiki_socket }};
 }

roles/nginx/defaults/main.yml

 ---
 letsencrypt_validation_dir: "/var/lib/letsencrypt"
-fastcgi_cache: false

roles/nginx/templates/nginx.conf.j2

@@ -72,11 +72,6 @@ http {
 
     index index.php index.html index.htm;
 
-{% if fastcgi_cache %}
-    fastcgi_cache_path /etc/nginx/wikicache levels=1:2 keys_zone=wiki:100m inactive=60m;
-    fastcgi_cache_key "$scheme$request_method$host$request_uri";
-
-{% endif %}
     access_log syslog:server=unix:/dev/log,nohostname,tag=nginx_http main;
 
     include snippets/sslsettings.conf;