Cleanup/drop support for Avenger/Nitrogen boards

The Avenger board was already deprecated and this clean-ups some
remaining references in the ci docs..

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

ci/device-testing.rst

@@ -148,8 +148,6 @@ Test devices are connected to their local LAVA worker and manged by LAVA server.
 
 * `How to deploy wic image on Raspberry Pi in LAVA <https://forum.ostc-eu.org/t/how-to-deploy-wic-image-on-raspberry-pi-in-lava/228>`_
 * `Adding Arduino Nano 33 BLE Board to LAVA Lab <https://forum.ostc-eu.org/t/adding-arduino-nano-33-ble-board-to-lava-lab/215>`_
-* `Adding Nitrogen Board to LAVA Lab <https://forum.ostc-eu.org/t/adding-nitrogen-board-to-lava-lab/192>`_
-* `Adding Avenger96 Board to LAVA Lab <https://forum.ostc-eu.org/t/adding-avenger96-board-to-lava-lab/46>`_
 
 References
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