If you are ready to do this with css, you can do it in the following clean way.
Add the following code in the current theme’s functions.php file
add_action( 'login_head', 'wpse_121687_hide_login' );
function wpse_121687_hide_login() {
$style="";
$style .= '<style type="text/css">';
$style .= '.login form{ display: none }';
$style .= '.login #nav a, .login #backtoblog a { display: none }';
$style .= '</style>';
echo $style;
}
or use the below way to enqueue the css file to the login page then use the css.
Add the below code in your current theme’s functions.php file
add_action( 'login_enqueue_scripts', 'wpse_121687_hide_login' );
function wpse_121687_hide_login() {
wp_register_style( 'hide-login', plugins_url( 'path to /css/hide-login.css' ) );
wp_enqueue_style( 'hide-login' );
}
And then add the css in the hide-login.css
file