I'm not very familiar with all this styling stuff, I discovered that by using 'inspect element'. I simplified the code and deleted all the unnecessary.

css

.sort-by select[test] 
{
    border: none;
}

Div with select

<div class="sort-by">
 <select id="sorting" name="sortby" class="cl-secondary" test=""> 
    <option>
      option 
    </option>
  </select>
</div>

I tested it here > https://www.tutorialspoint.com/online_css_editor.php

all other browsers show correctly and without the border

only Opera shows with a border

I'm on Win 8.1x64