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.