If you speak about the 'Comment' text area in the edit tab, you should remove it simply by overriding the template document_edit.xhtml (in nuxeo.war/incl/tabs) by your own file.

You just need to comment lines below:

 <table class="dataInput">
  <tbody>
    <tr>
     <td class="iconColumn" style="vertical-align:top;" />
     <td class="labelColumn">
       <h:outputText value="#{messages['label.comment']}" />
     </td>
     <td class="fieldColumn">
       <h:inputTextarea value="#{currentDocument.contextData['request/comment']}" />
     </td>
    </tr>
  </tbody>
 </table>

Else if you don't want override the default template a proper way IMO is to create your own layout (see http://doc.nuxeo.com/display/NXDOC/Layout+display) like this you keep the default behavior when you upgrade your version of Nuxeo.