samples: net: coap: Remove apps using raw mbedtls APIs

This removes these network sample applications
  samples/net/coaps_client
  samples/net/coaps_server

as they are using low level mbedtls APIs. You should use
preferably socket based or net-app based applications.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>