don't translate error messages twice

As @stof pointed out in #12164, error messages are already translated
when they are passed to the template.

src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_3_layout.html.twig

@@ -227,7 +227,7 @@
     {% if form.parent %}<span class="help-block">{% else %}<div class="alert alert-danger">{% endif %}
     <ul class="list-unstyled">
         {%- for error in errors -%}
-            <li><span class="glyphicon glyphicon-exclamation-sign"></span> {{ error.message|trans({}, translation_domain) }}</li>
+            <li><span class="glyphicon glyphicon-exclamation-sign"></span> {{ error.message }}</li>
         {%- endfor -%}
     </ul>
     {% if form.parent %}</span>{% else %}</div>{% endif %}