Stop enabling gzip_static in global Nginx server configuration

From a sysadmin I work with:

While at it, I took a quick look at /var/aegir/config/server_master/nginx.conf and noticed "gzip_static on". This seems to provide no benefit and costs a little performance. IMHO this should be moved to the per vhost files if they use that uncommon feature but it's certainly not a generic setting to enable globally.