Always use external tracker to cross reference external issues

What does this MR do?

I'm working towards removing the GitlabIssueTrackerService. With this MR I want to make sure that only external issue trackers can get called when there is an external issue. Technically this is already the case because it is enforced by app/services/system_notes/issuables_service.rb#L147. The change to external_issue_tracker just points out more clearly that it can't reach GitlabIssueTrackerService

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