minor #11282 removed part from xml which is not available in yaml and php code-blocks (OskarStark)

This PR was merged into the 3.4 branch.

Discussion
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removed part from xml which is not available in yaml and php code-blocks

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
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b258030 removed part from xml which is not available in yaml and php code-blocks

Author: javiereguiluz

service_container/optional_dependencies.rst

@@ -81,10 +81,6 @@ call if the service exists and remove the method call if it does not:
                 https://symfony.com/schema/dic/services/services-1.0.xsd">
 
             <services>
-                <service id="app.mailer">
-                <!-- ... -->
-                </service>
-
                 <service id="AppBundle\Newsletter\NewsletterManager">
                     <call method="setLogger">
                         <argument type="service" id="logger" on-invalid="ignore"/>