I have "fixed" this issue by simply hiding that button using a style.
I define a file admin.css which is loaded only when I'm in the Admin panel
In file:
functions.php:
if (is_admin()) {
/**
* Register my own admin.css for adding new styles
* Adds the function 'add_my_stylesheet' to the
* wp_enqueue_scripts action.
*/
function add_admin_stylesheet() {
echo '<link rel="stylesheet" href="' . get_bloginfo('stylesheet_directory') . '/css/admin.css" type="text/css" />';
}
add_action( 'admin_print_styles', 'add_admin_stylesheet' );
}
In file:
css/admin.css:
# Hide the "Attachment Post URL" from the Media Gallery
.urlpost {
visibility: hidden;
}
Hope this helps!