Remove CHANGED_PATH/URL CI variable code

What does this MR do?

This MR removes some code that has lived behind a feature flag that was never really turned on. It was an experimental implementation that ended up having some serious performance concerns, and we abandoned the approach.

It's essentially dead code, as it was never turned on for production use.

There is no documentation to remove - a little bit had been written prior but was removed when the feature flag was written in.

Once this is merged, we can close #214274 (closed) and #207383

Screenshots

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