Removes pending message count

The information was used only to log remaining messages on debug log.
It was checked on writing but updated prior to every call making the
verification meaningless.

lib/include/mqtt_client_priv.h

@@ -58,7 +58,6 @@ typedef struct mqtt_state {
     uint16_t pending_msg_id;
     int pending_msg_type;
     int pending_publish_qos;
-    int pending_msg_count;
 } mqtt_state_t;
 
 typedef struct {