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/client/credentials_test.cc b/test/cpp/client/credentials_test.cc
index 6fb24d7..18fcffe 100644
--- a/test/cpp/client/credentials_test.cc
+++ b/test/cpp/client/credentials_test.cc
@@ -45,8 +45,8 @@
  protected:
 };
 
-TEST_F(CredentialsTest, InvalidServiceAccountCreds) {
-  std::shared_ptr<Credentials> bad1 = ServiceAccountCredentials("", "", 1);
+TEST_F(CredentialsTest, InvalidGoogleRefreshToken) {
+  std::shared_ptr<Credentials> bad1 = GoogleRefreshTokenCredentials("");
   EXPECT_EQ(static_cast<Credentials*>(nullptr), bad1.get());
 }