Updates current tracking code to use Tracking.js

What does this MR do?

It simply moves forward with using the new Tracking interface instead of using stats.js.

This also makes it clear that we need a more concise and simple way to implement tracking into the wider system. I'll be spending some time trying to come up with a simpler and more consistent way to get tracking implemented at the various levels. We currently have https://gitlab.com/gitlab-org/telemetry/issues/87 open to address and track against that.

This here simply cleans up and will make EE track the same way that CE will. I'd like to hold on to this for a bit and will spike on a more holistic way to get this sort of thing in there.

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