boards/nrf6310: Remove nRF6310 board from RIOT

[crasbe]

Contribution description

When working on the documentation of the nRF chips (#20592), I discovered that there is a board called "NRF6310" which supports the nRF6310 board (sometimes apparently called nRFgo?) and the Mommosoft BLE DEVKIT.N. Both are based on the nRF51822 chip.

I am not sure since when the nRFgo board has not been sold anymore, but there are no references at any of the typical distributors and the Nordic documentation experiences bitrot as well.
Mommosoft has not been around anymore since ~2019 and the most recent blog entries about the "BLE DEVKIT.N" are from around 2014.
The nRF51822 is still sold and supported by Nordic, however it was set to NRND in 2022 and the Product Change Notice says that the distributors should remove it once stock is depleted (https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/4884/PDN_049.pdf).

In conclusion the nRF6310 is pretty much unavailable and has been for a long time. References to it are found in many Makefile.ci files because of insufficient memory, therefore I would consider it to be obsolete for RIOT.
The nRF51822 as well as the nRF51 DKs should stay supported, as they are still available for purchase.

In case that opinion is not shared by the RIOT devs, this PR can just be closed (or postponed).

Edit: I know that usually boards are removed after a deprecation notice, but this board has been dead for so long, it might not be necessary.

Testing procedure

There shouldn't be any CI errors, otherwise I wouldn't know how to test it?

Issues/PRs references

This is related to #20592.

Some information I found about the boards (note that the link to Mommosoft goes to the Web Archive..):
https://doc.riot-os.org/group__boards__nrf6310.html
https://developer.nordicsemi.com/nRF51_SDK/nRF51_SDK_v4.x.x/doc/html/group__nrf51__setups.html#nrf51_setup_nrf6310
https://web.archive.org/web/20191031043659/http://www.mommosoft.com/blog/category/nrf51822/

[Teufelchen1]

Thanks for taking out the trash 🗑️

I think I'll let another maintainer click on merge, just to be sure there is rough consensus.

[OlegHahm]

Thanks for the clean up. I agree that we probably don't need the deprecation for this board. However, IIRC we did the procedure for much older boards already in the past. The next release is around the corner: should we announce it as deprecreated in 2025.01 and remove it in 2025.04 then?
(From my perspective, instant removal would be also fine.)

[crasbe]

I think it would be the best solution to follow the procedure and mark it as deprecated in the upcoming release and remove it three months later. It's not that pressing IMO.

[miri64]

Regardless the outcome, its removal commit should be documented in the LOSTANDFOUND.md

[MrKevinWeiss]

ping :)

[MrKevinWeiss]

or why not... It looks like all the comments have been addressed.

[crasbe]

I rebased this PR, there were merge conflicts after #21146 in the LOSTANDFOUND.md (nothing bad, but git didn't want to automatically merge it in the right order).