update unsupported DS/DSi cores

[engdyn]

The old melonDS core is supported and already listed in the supported section.
I also added DeSmuME 2015 to the DSi section since it's a different core that is unsupported as well.

[greptile-apps]

Greptile Summary

Updates unsupported Nintendo DS/DSi core listings.

• Removes melonDS 2021 from Nintendo DS.
• Adds DeSmuME 2015 to Nintendo DSi.

Confidence Score: 5/5

The documentation update appears safe to merge.

No concrete incorrect or misleading classification remains.

_{Reviews (1): Last reviewed commit: "update unsupported DS/DSi cores" | Re-trigger Greptile}