Removed CoAP.NET submodule (temporarily)

Signed-off-by: Martin Evans <martindevans@gmail.com>
removed CoAP.NET

Signed-off-by: Martin Evans <martindevans@gmail.com>
Imported all the bits of CoAP.NET I need into Bastet (currently got a PR for CoAP.NET which will fix this).

Signed-off-by: Martin Evans <martindevans@gmail.com>