The second argument should be a function callback, not the actual message. You can remove the message completely by pasting this code into your theme’s functions.php
file:
add_filter( 'new_user_approve_welcome_message', 'wpse_76823_login_message' );
function wpse_76823_welcome_message( $message ) {
return '';
}
Or, you can replace the message by using this code:
add_filter( 'new_user_approve_welcome_message', 'wpse_76823_login_message' );
function wpse_76823_welcome_message( $message ) {
return "This is my new message";
}
If you need more help, post a comment below.