fix: remove unnecessary configuration due new mercure_publish() PHP function

[7-zete-7]

With the addition of the new PHP function mercure_publish() in php/frankenphp#1927, the use of the closure via the php:80 host is no longer necessary. The php:80 and the MERCURE_URL env can be removed.

[7-zete-7]

The actual symfony/mercure-bundle recipe still requires the MERCURE_URL env. I'm guessing it's not yet possible to prevent using the environment variable in this recipe, as other environments expect this behavior.