chore: Remmove class cass for getProjectId in GDCHCredentials (#1816)

Author: lqiu96

oauth2_http/javatests/com/google/auth/oauth2/DefaultCredentialsProviderTest.java

@@ -410,7 +410,7 @@ public void getDefaultCredentials_GdchServiceAccount() throws IOException {
 
     assertNotNull(defaultCredentials);
     assertTrue(defaultCredentials instanceof GdchCredentials);
-    assertEquals(GDCH_SA_PROJECT_ID, ((GdchCredentials) defaultCredentials).getProjectId());
+    assertEquals(GDCH_SA_PROJECT_ID, defaultCredentials.getProjectId());
     assertEquals(
         GDCH_SA_SERVICE_IDENTITY_NAME,
         ((GdchCredentials) defaultCredentials).getServiceIdentityName());
@@ -423,7 +423,7 @@ public void getDefaultCredentials_GdchServiceAccount() throws IOException {
         ((GdchCredentials) defaultCredentials).createWithGdchAudience(GDCH_SA_API_AUDIENCE);
     assertNotNull(defaultCredentials);
     assertTrue(defaultCredentials instanceof GdchCredentials);
-    assertEquals(GDCH_SA_PROJECT_ID, ((GdchCredentials) defaultCredentials).getProjectId());
+    assertEquals(GDCH_SA_PROJECT_ID, defaultCredentials.getProjectId());
     assertEquals(
         GDCH_SA_SERVICE_IDENTITY_NAME,
         ((GdchCredentials) defaultCredentials).getServiceIdentityName());