Fix typo: s/no_acceptable/not_acceptable

in XMPPError.Condition
Reply correct error code on XEP-65 Stream rejection

this was previously erroneously changed with
197548b548c0436108cb20f33859172a70d0bf8f 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.