net: zoap: Remove unused helper function

Commit "net: zoap: Fix memory overflow issue" fixed and implemented
./well-known/core response in a different way, so this api is not needed
anymore.

Change-Id: I8f945fb5842028be50ecfdef95cbe5da3189a538
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>