Remove Genericons Helper CSS from twentyfourteen theme

First of all, you’re trying to deregister a stylesheet that doesn’t exist. Here is how the genericon stylesheet is enqueued in twenty fourteen

wp_enqueue_style( 'genericons', get_template_directory_uri() . '/genericons/genericons.css', array(), '3.0.2' );

You are trying to dequeue the stylesheet file name. Here is the wp_enqueue_style() function

wp_enqueue_style( $handle, $src, $deps, $ver, $media );

The important part here is the $handle

$handle (string) (required) Name used as a handle for the stylesheet.

You should use the $handle if you need to dequeue a stylesheet. Another thing to remember, you are running a child theme (I really hope so). Child theme functions are loaded first, so you will need to give your deregister function a priority of more than 10, otherwise your functions will not work.

Here is the working function

function dequeue_my_css() {
  wp_dequeue_style('genericons');
  wp_deregister_style('genericons');
}
add_action('wp_enqueue_scripts','dequeue_my_css', 100);

In the twenty fourteen theme, this will however break the complete styling of your theme. Unfortunately the author has made the main stylesheet dependant on the genericon stylesheet, so if you dequeue the genericon stylesheet, you “dequeue” the main stylesheet as well.