message: build(aio): remove "pure" property from pipe template
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  Closes
diff: {% block details %}
  {% include "includes/description.html" %}
  -<div class="pipe-properties">
  - {% if doc.pipeOptions.pure == "true" %}<label class="pipe-type-label pure">pure</label>{% endif %}
  -</div>
  {% endblock %}
  