Remove unused attribute from ActivityService

What does this MR do?

In gitlab-foss!25005 (merged) the debug log for user activity was removed and now the activity attribute is not needed anymore.

This was discovered in !21725 (merged) but to keep changes small I decided to address it in a separate MR

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (There is no actual change. It just removes unused code)
• [-] 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

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