DOC: ssl: remove prefer-server-ciphers documentation

Commit 3c4bc6e1 removed the prefer-server-ciphers statement.
Therefore, the statement should also be removed from the documentation.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 50c6bb1..96b4796 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1533,7 +1533,7 @@
 
         listen http_https_proxy
             bind :80
-            bind :443 ssl crt /etc/haproxy/site.pem prefer-server-ciphers
+            bind :443 ssl crt /etc/haproxy/site.pem
 
   See also : "source", "option forwardfor", "unix-bind" and the PROXY protocol
              documentation, and section 5 about bind options.
@@ -6958,11 +6958,6 @@
   cannot be enabled using any configuration option. See also "force-tls*",
   and "force-sslv3".
 
-prefer-server-ciphers
-  This setting is only available when support for OpenSSL was built in. It
-  tells the SSL/TLS layer that our set of cipher algorithms is preferred over
-  the client's ones.
-
 ssl
   This setting is only available when support for OpenSSL was built in. It
   enables SSL deciphering on connections instanciated from this listener. A