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

.github/ci/matrix.json

@@ -61,7 +61,6 @@
     "x86-64-bmi2",
     "x86-64-avxvnni",
     "x86-64-avx512",
-    "x86-64-vnni256",
     "x86-64-vnni512",
     "x86-64-avx512icl",
     "apple-silicon",
@@ -105,12 +104,6 @@
         "os": "macos-14"
       }
     },
-    {
-      "binaries": "x86-64-vnni256",
-      "config": {
-        "os": "macos-14"
-      }
-    },
     {
       "binaries": "x86-64-vnni512",
       "config": {
@@ -135,12 +128,6 @@
         "os": "macos-15-intel"
       }
     },
-    {
-      "binaries": "x86-64-vnni256",
-      "config": {
-        "os": "macos-15-intel"
-      }
-    },
     {
       "binaries": "x86-64-vnni512",
       "config": {
@@ -189,12 +176,6 @@
         "os": "windows-11-arm"
       }
     },
-    {
-      "binaries": "x86-64-vnni256",
-      "config": {
-        "os": "windows-11-arm"
-      }
-    },
     {
       "binaries": "x86-64-vnni512",
       "config": {