Delete old CoAP implementation

It seems that this implementation of CoAP in Contiki is no longer maintained in favor of the `er-coap` implementation. This commit removes the code to prevent further bit-rot.

You may also read between the lines here: I spend a good deal of time trying to get this example to work on the CC2538. After adding a `sbrk()` function and editing the linker script to get it to compile, I dove into the RFC to figure out why the REST server wasn't working with Copper (Firefox CoAP plugin). Turns out this implementation supports some old version of the CoAP spec which is quite different from the RFC.