[vcl] don't explicitly hash the host header

Hashing `req.http.host`/`client.ip` is already handled by the [built-in
vcl](https://github.com/varnishcache/varnish-cache/blob/6.0/bin/varnishd/builtin.vcl#L86)
so there's no need to repeat it explicitly.

It's also a bit confusing as `req.url` is not explicitly handled, even
though it's a more important hash input than the host.

note: all versions have been changed for the sake of consistency
but both the 4.x and 5.x series have been EOL'd a (long) while ago and users
should be encouraged to upgraded as soon as possible.

Author: Guillaume Quintard

app/code/Magento/PageCache/etc/varnish4.vcl

@@ -121,13 +121,6 @@ sub vcl_hash {
         hash_data(regsub(req.http.cookie, "^.*?X-Magento-Vary=([^;]+);*.*$", "\1"));
     }
 
-    # For multi site configurations to not cache each other's content
-    if (req.http.host) {
-        hash_data(req.http.host);
-    } else {
-        hash_data(server.ip);
-    }
-
     if (req.url ~ "/graphql") {
         call process_graphql_headers;
     }

app/code/Magento/PageCache/etc/varnish5.vcl

@@ -122,13 +122,6 @@ sub vcl_hash {
         hash_data(regsub(req.http.cookie, "^.*?X-Magento-Vary=([^;]+);*.*$", "\1"));
     }
 
-    # For multi site configurations to not cache each other's content
-    if (req.http.host) {
-        hash_data(req.http.host);
-    } else {
-        hash_data(server.ip);
-    }
-
     # To make sure http users don't see ssl warning
     if (req.http./* {{ ssl_offloaded_header }} */) {
         hash_data(req.http./* {{ ssl_offloaded_header }} */);

app/code/Magento/PageCache/etc/varnish6.vcl

@@ -122,13 +122,6 @@ sub vcl_hash {
         hash_data(regsub(req.http.cookie, "^.*?X-Magento-Vary=([^;]+);*.*$", "\1"));
     }
 
-    # For multi site configurations to not cache each other's content
-    if (req.http.host) {
-        hash_data(req.http.host);
-    } else {
-        hash_data(server.ip);
-    }
-
     # To make sure http users don't see ssl warning
     if (req.http./* {{ ssl_offloaded_header }} */) {
         hash_data(req.http./* {{ ssl_offloaded_header }} */);