Clean up attention request related system note metadata

We've removed attention request related backend code, but we have existing system_note_metadata with attention_requested state.

We don't have to clean these up as it's used for displaying icons.

I haven't removed following lines yet, but I think we could clean up these metadata and the following code.

• https://gitlab.com/gitlab-org/gitlab/blob/6e2f8d14ff77ad3f540b9b2ae0961b5f59fd9148/app/models/system_note_metadata.rb#L27
• https://gitlab.com/gitlab-org/gitlab/blob/6e2f8d14ff77ad3f540b9b2ae0961b5f59fd9148/app/helpers/system_note_helper.rb#L45

We could just leave them in place as a legacy metadata and be done with it though.

Related to !95446 (merged) and #366442 (closed)

Testing

Run the package and qa job as a regression test. @jay_mccure