Revert adding productivity analytics docs

What does this MR do?

Since the Productivity Analytics feature might not get deployed in 12.2 we need to remove the docs. This MR reverts the changes from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31460 where we added docs for the new Productivity Analytics feature.

/cc @valexieva as requested in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31460#note_203987991

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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