Revert "Bind metrics on :: too"

This reverts commit 536c85619f.

Looks like binding metrics on IPv6 (`::`) fails with an error:

socket.gaierror: [Errno -2] Name does not resolve

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -152,7 +152,8 @@ listeners:
 {% if matrix_synapse_metrics_enabled %}
   - type: metrics
     port: {{ matrix_synapse_metrics_port }}
-    bind_addresses: ['::']
+    bind_addresses:
+      - '0.0.0.0'
 {% endif %}
 
 {% if matrix_synapse_federation_enabled and matrix_synapse_tls_federation_listener_enabled %}