How to disable or hide “collapse menu”

Here is a simple css display: none function, it just adds some css in the html, if there are more stuff that you want to hide i recommend you to add a new css-file with the function admin_enqueue_scripts

function wpse_remove_collapse() {
   echo '<style type="text/css">#collapse-menu { display: none; visibility: hidden; }</style>';
}
add_action('admin_head', 'wpse_remove_collapse');