Remove redundant sentence

Spotted by David Johnston

doc/src/sgml/protocol.sgml

@@ -272,7 +272,7 @@
         Kerberos specification) with the server.  If this is
         successful, the server responds with an AuthenticationOk,
         otherwise it responds with an ErrorResponse. This is no
-        longer supported. This is not supported any more.
+        longer supported.
        </para>
       </listitem>
      </varlistentry>