Remove feature flag usage_activity_by_stage

What does this MR do?

Remove feature flag usage_activity_by_stage

• It was there because most counters would time out see main issue https://gitlab.com/gitlab-org/telemetry/-/issues/308
• After the batch counters & a 3 months of indexing improvements, we have have now zero broken counters in gitlab.com scale https://gitlab.com/gitlab-org/gitlab/-/issues/208233
• Btw it was moved to default ON 2 releases ago - and was released to self-managed and gitlab.com from 12.9 onwards.

# checking the chatops for the feature list shows it's disabled on igtlab.com

/chatops run feature list --match usage
GitLab ChatopsAPP  07:16
Enabled Features
These features are enabled:
  0 enabled features on gitlab.com
Disabled Features
These features are disabled:
  0 disabled features on gitlab.com

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

Related to #213963 (closed)