WvStream destructor: remove.

WvStream destructor: remove.

Having a destructor on the base class is pretty unhelpful; WvStreams
themselves are disposable, but mostly there's no need to dispose them.
WvSockStream might need to be disposed, but if you forget, the Socket object
will do its best to handle things for you anyway; doing something from
~WvStream() is no help and actually makes things worse, since a non-critical
finalizer gets enabled for streams like WvLog, reputedly slowing down
garbage collection.

So let's leave out the destructor altogether in the base class.  If another
class really needs a destructor someday - and currently none do - then we
can always add one.