Remove py312 vanilla test (#831)

Author: oulgen

.github/matrix.json

@@ -29,15 +29,6 @@
       "dtype-asserts": true,
       "expecttest-accept": true
     },
-    {
-      "runner": "linux.g5.4xlarge.nvidia.gpu",
-      "python-version": "3.12",
-      "ref-eager": false,
-      "image": "nvidia/cuda:12.8.1-devel-ubuntu24.04",
-      "runtime-version": "cu128",
-      "container-options": "--gpus all",
-      "alias": "a10g"
-    },
     {
       "runner": "linux.g5.4xlarge.nvidia.gpu",
       "python-version": "3.12",