Remove x86-64-vnni256 target
When vnni256 was first introduced #3038 it was very slightly faster than vnni512
on some machines. We have since sped up vnni512 significantly (#4796 over 10%
alone, #6139, and probably others I'm forgetting). Since any machine that can
run vnni256 can run vnni512 this arch is no longer useful.
Note, x86-64-avxvnni still covers targets that don't have AVX512 but do have
VNNI on 128- and 256-bit vectors.
closes https://github.com/official-stockfish/Stockfish/pull/6340
No functional change