Removed unused link definition

Author: Thomas Berends

mailer.rst

@@ -781,5 +781,4 @@ environment:
 .. _`Inky`: https://foundation.zurb.com/emails.html
 .. _`S/MIME`: https://en.wikipedia.org/wiki/S/MIME
 .. _`OpenSSL PHP extension`: https://php.net/manual/en/book.openssl.php
-.. _`Let's Encrypt project`: https://letsencrypt.org/
 .. _`PEM encoded`: https://en.wikipedia.org/wiki/Privacy-Enhanced_Mail