title: yii2 remove optgroup from dropdownlist
body: <p>How can i remove the optgroup from a checkbox field</p>
  
  <pre><code>&lt;?= $form-&gt;field($model, 'survey_type')-&gt;dropDownList([$surveyTypeList],['prompt'=&gt;'Select Survey Type','id'=&gt;'Survey_Type_dropdown']) ?&gt;
  </code></pre>
  
  <p>provides following html</p>
  
  <pre><code>&lt;label class="control-label" for="Survey_Type_dropdown"&gt;Type&lt;/label&gt;
  &lt;select id="Survey_Type_dropdown" class="form-control" name="Surveys[survey_type]"&gt;
  &lt;option value=""&gt;Select Survey Type&lt;/option&gt;
  &lt;optgroup label="0"&gt;
  &lt;option value="2D"&gt;2D&lt;/option&gt;
  &lt;option value="3D"&gt;3D&lt;/option&gt;
  &lt;/optgroup&gt;
  &lt;/select&gt;
  
  &lt;div class="help-block"&gt;&lt;/div&gt;
  </code></pre>