Remove dup metrics_external_dashboard_url

What does this MR do?

Fix master, remove dupe method

Offenses:

app/helpers/projects_helper.rb:352:3: W: Lint/DuplicateMethods: Method ProjectsHelper#metrics_external_dashboard_url is defined at both app/helpers/projects_helper.rb:324 and app/helpers/projects_helper.rb:352.
  def metrics_external_dashboard_url
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

11859 files inspected, 1 offense detected

https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/221741576

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11830

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team