Remove sharding from custom_root_test.

This test only takes around 30s on most hardware platforms, it does not need 10 shards.

PiperOrigin-RevId: 694243316

Author: hawkinsp

tests/BUILD

@@ -416,11 +416,6 @@ jax_multiplatform_test(
 jax_multiplatform_test(
     name = "custom_root_test",
     srcs = ["custom_root_test.py"],
-    shard_count = {
-        "cpu": 10,
-        "gpu": 10,
-        "tpu": 10,
-    },
 )
 
 jax_multiplatform_test(