tirar border do top

<nav class="art-nav">
   <div id="barra_letras_dir"><a href="#"> Login</a>&nbsp;&nbsp;/&nbsp;&nbsp;Criar Conta </div>
   <div id="barra_letras_esq"> Ajuda &amp; Informa&ccedil;&otilde;es&nbsp;/&nbsp;Convidar </div>
 </nav>
 <div class="log_reg"><p>&nbsp;</p>
       <form action="" method="post">
        <fieldset>
               <label>
               <span class="">Nickname:</span>
               <input type="text" name="nome" />
               </label>
           </fieldset>
           <fieldset>
<label>
               <span class="">Password:</span>
               <input type="password" name="password"/>
               </label>
<input type="submit" value="Iniciar Sessão" class="send" />

           </fieldset>
</form>

 </div>

.log_reg{
width:300px;
height:auto;
border-radius: 0px 0px 5px 5px;
border:1px #FFFFFF solid;
background-color:#57cbd1;
margin-top:0px;
margin-left:64%;
position: absolute;
z-index: 1000;
}