Contact form opt-out line should be excluded from admin-sent and sender-copy e-mails

I just noticed that when an admin sends an email to a user, it has this message:

Brad Landis (site/user/1) has sent you a message via your
contact form (site/user/12/contact) at siteName.

If you don't want to receive such e-mails, you can change your settings at
site/user/12.

Ironically, even if you change the settings, the admin can still contact you, which was the case for this email. If the admin user sends the email, this message should not be included.