Do not try to load certificates

Seems like we unintentionally removed the mounting of certificates
(the `/matrix-config` mount) as part of splitting the playbook into
roles in 51312b8250.

It appears that those certificates weren't necessary for coturn to
funciton though, so we might just get rid of the configuration as well.

roles/matrix-coturn/templates/turnserver.conf.j2

@@ -1,10 +1,6 @@
 use-auth-secret
 static-auth-secret={{ matrix_coturn_turn_static_auth_secret }}
 realm=turn.{{ hostname_matrix }}
-cert=/matrix-config/{{ hostname_matrix }}.tls.crt
-pkey=/matrix-config/{{ hostname_matrix }}.tls.key
-dh-file=/matrix-config/{{ hostname_matrix }}.tls.dh
-cipher-list="HIGH"
 min-port={{ matrix_coturn_turn_udp_min_port }}
 max-port={{ matrix_coturn_turn_udp_max_port }}
 external-ip={{ matrix_coturn_turn_external_ip_address }}
@@ -12,3 +8,4 @@ log-file=stdout
 pidfile=/var/tmp/turnserver.pid
 userdb=/var/tmp/turnserver.db
 no-cli
+prod