removed non-functional 'change option'

src/app/panels/histogram/editor.html

   <div class="row-fluid">
     <div class="span2">
       <label class="small">Mode</label>
-      <select ng-change="set_refresh(true)" class="input-small" ng-model="panel.mode" ng-options="f for f in ['count','min','mean','max','total','change']"></select>
+      <select ng-change="set_refresh(true)" class="input-small" ng-model="panel.mode" ng-options="f for f in ['count','min','mean','max','total']"></select>
     </div>
     <div class="span2">
       <label class="small">Time Field</label>