Remove experimental warning

src/Packagist/WebBundle/Resources/views/Email/update_failed.txt.twig

@@ -1,8 +1,4 @@
 {% autoescape false -%}
-> Disclaimer: These email notifications are in experimental stage, if you find
-> that they look strange or contain false or misleading information please
-> reply to this email to let us know.
-
 The {{ package.name }} package of which you are a maintainer has
 failed to update due to invalid data contained in your composer.json.
 Please address this as soon as possible since the package stopped updating.