Remove NULL_CHANNEL and Code Warrior 8 workaround

[noloader]

BufferedTransformation includes the following:

class BufferedTransformation : public Algorithm, public Waitable
{
public:
    // placed up here for CW8, same as DEFAULT_CHANNEL, for backwards compatibility
    static const std::string &NULL_CHANNEL;
    ...
};

NULL_CHANNEL dates back to the Crypto++ 4.0 (and earlier) days. In addition, Code Warrior 8 is older than the .Net compilers we support. Finally, NULL_CHANNEL contributes to Static Initialization Order Fiasco.

For the upcoming Crypto++ 6.0 release, we will remove the deprecated channel name and Code Warrior support.

[noloader]

Cleared at Commit 5fb2f5d45b9bb2cd.