Fixing a compilation error in integ tests (#760)

Author: hiranya911

test/integration/setup.ts

@@ -19,7 +19,7 @@ import fs = require('fs');
 import minimist = require('minimist');
 import path = require('path');
 import {random} from 'lodash';
-import { Credential, GoogleOAuthAccessToken, Certificate } from '../../src/auth/credential';
+import { Credential, GoogleOAuthAccessToken } from '../../src/auth/credential';
 
 /* tslint:disable:no-var-requires */
 const chalk = require('chalk');
@@ -117,10 +117,6 @@ class CertificatelessCredential implements Credential {
   public getAccessToken(): Promise<GoogleOAuthAccessToken> {
     return this.delegate.getAccessToken();
   }
-
-  public getCertificate(): Certificate | null {
-    return null;
-  }
 }
 
 /**