Use internal issue url unless external tracker

What does this MR do?

TL;DR: This MR should not change the way we generate issue_url, just the underlying code so it is not possible to call GitlabIssueTrackerService anymore.

In !28219 (closed) I tried to remove the GitlabIssueTrackerService. It is the default issue tracker service but it looks like it is not used anymore. There are still some places where it could be called. For example when IssuesHelper.url_for_issue is called with an internal issue and without the internal option. Except for specs, I couldn't find a place where this would actually happen but just to be safe, this MR changes it to use url_helpers over GitlabIssueTrackerService when there is no external issue tracker.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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