Remove experimental MXID value from user notification preferences

* since we're not quite ready for matrix notifications yet, this wasn't
  supposed to be added to the code

invenio_config_tuw/users/templates/notifications_settings.html

@@ -37,7 +37,6 @@ details.
               {#- change: create vars for other fields -#}
               {%- set enabled_field = form.enabled %}
               {%- set secondary_email_field = form.secondary_email %}
-              {%- set matrix_user_id_field = form.matrix_user_id %}
               {%- set notif_enabled = current_user.preferences.get("notifications", {}).get("enabled", True) %}
 
               <div class="field">