minor #12185 Remove unused link (OskarStark)

This PR was merged into the 3.4 branch.

Discussion
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Remove unused link

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Commits
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b4f3a02 Remove unused link

Author: wouterj

deployment/proxies.rst

@@ -93,5 +93,4 @@ In this case, you'll need to set the header ``X-Forwarded-Proto`` with the value
     $response = $kernel->handle($request);
 
 .. _`security groups`: http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-security-groups.html
-.. _`RFC 7239`: http://tools.ietf.org/html/rfc7239
 .. _`CloudFront`: https://en.wikipedia.org/wiki/Amazon_CloudFront