Sorry i didn't understand well at first. In order to avoid the button, you actually have a couple of ways:
applying some css to your button so that it becomes transparent, leaving just the icon visible:
background: transparent; border: none; padding: 0;
substitute rails submit with jquery (or similar) submit. Something like:
<span class="glyphicon"></span>
$('span.glyphicon').click(function(){ $('#my_form').submit()});
Until rails 3 one could use the link_to_function or link_to_remote helpers to call a js function, but since rails 4 this is not possible anymore. Take a look also Rails 3 submit form with link, and here Rails, how do you submit a form with a text link?
hope this helps