There is no setting for this, but it would be easy enough to achieve by tweaking the PHP code.
Open sc_admin/includes/contact/posted.php
From about line 232 you'll find a block of code that starts with a line that says:
if ($website != "") ...
This block continues to about line 264 and ends with the line that says:
$notify_body = ...
Comment out or delete all of that code.
Here is an amended version of that last line that you should add to your PHP after the commented/deleted code:
$notify_body = "\n$name_full has sent you a message.\n\nView all messages received in your online inbox.\n\n----------------------------------------$show_joinlist";
This should achieve the desired effect.
You might also like to add the URL of your simpleContact admin into that message, so you get a link in the email.
Please note that should you update simpleContact in future, you will have to re-apply any modifications you make.