Credentials cleanup:

- Removing service_accounts credentials. These credentials just have
  drawbacks compared to service_account_jwt_access credentials, notably
  in terms for security.
- Renaming Google specific credentials with a Google prefix for C and
  C++. This should be done as well for wrapped languages.
diff --git a/test/cpp/interop/client_helper.cc b/test/cpp/interop/client_helper.cc
index abc14ae..7093463 100644
--- a/test/cpp/interop/client_helper.cc
+++ b/test/cpp/interop/client_helper.cc
@@ -64,16 +64,6 @@
 namespace grpc {
 namespace testing {
 
-namespace {
-std::shared_ptr<Credentials> CreateServiceAccountCredentials() {
-  GPR_ASSERT(FLAGS_enable_ssl);
-  grpc::string json_key = GetServiceAccountJsonKey();
-  std::chrono::seconds token_lifetime = std::chrono::hours(1);
-  return ServiceAccountCredentials(json_key, FLAGS_oauth_scope,
-                                   token_lifetime.count());
-}
-}  // namespace
-
 grpc::string GetServiceAccountJsonKey() {
   static grpc::string json_key;
   if (json_key.empty()) {
@@ -86,7 +76,7 @@
 }
 
 grpc::string GetOauth2AccessToken() {
-  std::shared_ptr<Credentials> creds = CreateServiceAccountCredentials();
+  std::shared_ptr<Credentials> creds = GoogleComputeEngineCredentials();
   SecureCredentials* secure_creds =
       dynamic_cast<SecureCredentials*>(creds.get());
   GPR_ASSERT(secure_creds != nullptr);
@@ -107,14 +97,10 @@
   snprintf(host_port, host_port_buf_size, "%s:%d", FLAGS_server_host.c_str(),
            FLAGS_server_port);
 
-  if (test_case == "service_account_creds") {
-    std::shared_ptr<Credentials> creds = CreateServiceAccountCredentials();
-    return CreateTestChannel(host_port, FLAGS_server_host_override,
-                             FLAGS_enable_ssl, FLAGS_use_prod_roots, creds);
-  } else if (test_case == "compute_engine_creds") {
+  if (test_case == "compute_engine_creds") {
     std::shared_ptr<Credentials> creds;
     GPR_ASSERT(FLAGS_enable_ssl);
-    creds = ComputeEngineCredentials();
+    creds = GoogleComputeEngineCredentials();
     return CreateTestChannel(host_port, FLAGS_server_host_override,
                              FLAGS_enable_ssl, FLAGS_use_prod_roots, creds);
   } else if (test_case == "jwt_token_creds") {