bluetooth: remove @return doc for void functions

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

include/bluetooth/uuid.h

@@ -1877,8 +1877,6 @@ bool bt_uuid_create(struct bt_uuid *uuid, const uint8_t *data, uint8_t data_len)
  *  @param uuid Bluetooth UUID
  *  @param str pointer where to put converted string
  *  @param len length of str
- *
- *  @return N/A
  */
 void bt_uuid_to_str(const struct bt_uuid *uuid, char *str, size_t len);