Remove Nitrogen from supported targets

Based on the decision taken on the Thursday engineering call by the team (on 21st of July, 2022), Nitrogen will be no any longer a supported target in Oniro. It requires some additional tasks around it. This epic can gather them as a container. The epic scope:

• removing from the existed code (e.g. from the flavours)
• removing from the docs (also all dependencies there related to the board)
• removing from CI and the Lava instance