test(spanner): update instance admin iam test to skip service account check (#9747)

Author: arunmoezhi

google/cloud/spanner/admin/integration_tests/instance_admin_integration_test.cc

@@ -220,10 +220,6 @@ TEST_F(InstanceAdminClientTest, InstanceIam) {
   ASSERT_FALSE(in.project_id().empty());
   ASSERT_FALSE(in.instance_id().empty());
 
-  ASSERT_FALSE(internal::GetEnv("GOOGLE_CLOUD_CPP_SPANNER_TEST_SERVICE_ACCOUNT")
-                   .value_or("")
-                   .empty());
-
   auto actual_policy = client_.GetIamPolicy(in.FullName());
   if (Emulator() &&
       actual_policy.status().code() == StatusCode::kUnimplemented) {