doc: Remove incorrect SCRAM protocol documentation

The documentation claimed that one should send
"pg_same_as_startup_message" as the user name in the SCRAM messages, but
this did not match the actual implementation, so remove it.

doc/src/sgml/protocol.sgml

@@ -1529,9 +1529,7 @@ that the client sends in the <structname>client-first-message</>. The user name
 that was already sent in the startup message is used instead.
 <productname>PostgreSQL</> supports multiple character encodings, while SCRAM
 dictates UTF-8 to be used for the user name, so it might be impossible to
-represent the PostgreSQL user name in UTF-8. To avoid confusion, the client
-should use <literal>pg_same_as_startup_message</literal> as the user name in the
-<structname>client-first-message</>.
+represent the PostgreSQL user name in UTF-8.
   </para>
 
   <para>