Disable support for legacy deprecated traces

What does this MR do?

This MR is decoupled from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23358, originally proposed by @ayufan.

The idea here, is to disable it today. Document what needs to be executed in order to migrate all files, and remove support for non-migrated stuff with %12.0, thus removing feature flags.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer