drivers/dht: remove useless auto_init declaration

drivers/include/dht.h

@@ -77,11 +77,6 @@ typedef struct {
  */
 typedef dht_t dht_params_t;
 
-/**
- * @brief auto-initialize all configured DHT devices
- */
-void dht_auto_init(void);
-
 /**
  * @brief initialize a new DHT device
  *