Remove api_activity_logging feature flag

What does this MR do?

It removes the api_activity_logging feature flag, enabling last activity logging on the REST API

The feature is documented on the API level and on instance statistics

How to test on Staging

1. Remove the feature flag by running /chatops run feature delete api_activity_logging --staging
2. Create a test user on staging with an API token
3. Wait until the next day.
4. Go to staging.gitlab.com/admin/users
5. Search for the test user
6. Make sure Last activity is not the date of today
7. Make an API call with the test user token curl -H "Authorization: Bearer TEST_USER_TOKEN" https://staging.gitlab.com/api/v4/projects
8. Reload the admin users page and check that Last activity was updated to the date of today

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