coap_block.c: Fix returning 2.03 for ETag match

Tag server returned data with a digest computed ETag option value so
requests with an ETag option can return 2.03 if there is a match.

The size of bytes hash that is computed for the ETag can be overwritten by
localy defining COAP_ETAG_MAX_BYTES to be a value between 1 and 8 bytes
inclusive.  It is set to 4 by default.

include/coap3/coap_net_internal.h

@@ -163,7 +163,6 @@ struct coap_context_t {
   uint32_t csm_timeout_ms;         /**< Timeout for waiting for a CSM from
                                            the remote side. */
   uint32_t csm_max_message_size;   /**< Value for CSM Max-Message-Size */
-  uint64_t etag;                   /**< Next ETag to use */
 
 #if COAP_SERVER_SUPPORT
   coap_cache_entry_t *cache;       /**< CoAP cache-entry cache */