cppcheck: remove unused functions

Unused functions removed:
    SetupHeader()
    OTClientConnection::AddToOutputList()
    OTClientConnection::GetNextInputMessage()
    OTClientConnection::GetNextOutputMessage()
    OTClientConnection::ProcessBuffer()
    OTServer::VerifyBasketAccountID()

Functions removed in consequence:
    OTClientConnection::ReadBytesIntoBuffer()
    OTClientConnection::ProcessMessage()
    OTClientConnection::ProcessType1Cmd()
    OTClientConnection::AddToInputList()
OTClientConnection: remove declaration of unimplemented method