[JENKINS-37518] Fix api url problem

New GitLab Api Client will automatically add api/v3 so we do not need to add api/v3 in config.
Maybe this config node can be delete in the future.

src/main/webapp/help/realm/gitlab-api-uri-help.html

@@ -2,14 +2,14 @@
     <p>
         Normally this value stays as-is. If you are using <b><a href="https://enterprise.gitlab.com">GitLab Enterprise</a></b>
         then you should enter the URI to the <strong>API</strong> root of your GitLab installation
-        (e.g. <b>https://gitlab.company.com/api/v3</b>).
+        (e.g. <b>https://gitlab.company.com</b>. The plugin will automatically add "api/v3" suffix for you.).
     <p/>
 
     Notes:
 
     <ol>
         <li>The <b>https://</b> / <b>http://</b> part needs to be specified.</li>
         <li>There should <strong>not</strong> be any trailing slash (/) </li>
-        <li>For GitLab Enterprise, the API URI is <em>usually</em> the same as the Web URI, with <b>/api/v3</b> appended.</li>
+        <li>For GitLab Enterprise, the API URI is <em>usually</em> the same as the Web URI.</li>
     </ol>
 </div>