Remove canutils/libuavcan and examples/uavcan. libuavcan has not built for a...

Remove canutils/libuavcan and examples/uavcan.  libuavcan has not built for a year or so.  The basic problem is that as NuttX advances, the old frozen versino of libuavcan has become absolute because it violates the portable POSIX OS interface.  No one is maintaining the port so there is no alternative but to remove it.