Reply correct error code on XEP-65 Stream rejection

this was previously erroneously changed with
197548b548 from 'forbidden' to
'not-acceptable'. We now change it back to 'forbidden'.

Also delete FileTransferNegotiator.rejectStream(StreamInitation) because
it is dead code and add some comments and javadoc.

smack-extensions/src/main/java/org/jivesoftware/smackx/bytestreams/socks5/Socks5BytestreamManager.java

@@ -703,6 +703,9 @@ public final class Socks5BytestreamManager implements BytestreamManager {
     /**
      * Responses to the given packet's sender with a XMPP error that a SOCKS5 Bytestream is not
      * accepted.
+     * <p>
+     * Specified in XEP-65 5.3.1 (Example 13)
+     * </p>
      * 
      * @param packet Packet that should be answered with a not-acceptable error
      * @throws NotConnectedException