doc: nfc: remove references to missing symbols

Fix doxygen warning about non-existing symbols `nfc_t2t_format_internal`
and `nfc_fixes.h`, by removing @ref's.

Signed-off-by: Fabio Utzig <utzig@apache.org>

nfc/include/nfc_t2t_lib.h

@@ -203,7 +203,6 @@ int nfc_t2t_payload_raw_set(const u8_t *payload,
  *
  * @note When modifying the internal bytes, remember that they must be
  *	 consistent with the NFC hardware register settings
- *	 (see @ref nfc_t2t_format_internal).
  *
  * @param data Pointer to the memory area containing the data.
  * @param data_length Size of the data in bytes.

nfc/include/nfc_t4t_lib.h

@@ -42,8 +42,7 @@
  *
  * @note If you are using nRF52832 chip (in IC rev. Engineering B or
  * Engineering C) or if You are using nRF52840 chip (in IC rev. Engineering A,
- * B or C) library will use TIMER 4 to apply workarounds for the anomalies
- * listed in @ref nfc_fixes.h
+ * B or C) library will use TIMER 4 to apply workarounds for the anomalies.
  */
 
 #include <zephyr/types.h>