doc: delete unused/misplaced link

The link to adafruit pinouts in the doc for arduino
nano 33 ble was probably copypasta. It probably hasn't
been noticed so far because it's not referenced in that
file.

Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>

boards/arm/arduino_nano_33_ble/doc/index.rst

@@ -214,9 +214,6 @@ References
 .. _BLE SENSE:
     https://store.arduino.cc/products/arduino-nano-33-ble-sense
 
-.. _pinouts:
-    https://learn.adafruit.com/introducing-the-adafruit-nrf52840-feather/pinouts
-
 .. _schematic:
     https://content.arduino.cc/assets/NANO33BLE_V2.0_sch.pdf