[Feature flag] Remove tribute_autocomplete

What

We've decided to not move forward with tribute, so this issue entails removing the :tribute_autocomplete feature flag and relevant code.

Remove the `:tribute_autocomplete` feature flag

Owners

• Team: groupproject management
• Most appropriate slack channel to reach out to: #s_plan
• Best individual to reach out to: @cngo

Expectations

What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab project
• gitlab-org/gitlab-com groups
• ...

Roll Out Steps

• Confirm that QA tests pass with the feature flag enabled (if you're unsure how, contact the relevant stable counterpart in the Quality department)
• Enable on staging (/chatops run feature set tribute_autocomplete true --staging)
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour (/chatops run feature set --project=gitlab-org/gitlab tribute_autocomplete true)
• Coordinate a time to enable the flag with the SRE oncall and release managers
  • In #production mention @sre-oncall and @release-managers. Once an SRE on call and Release Manager on call confirm, you can proceed with the rollout
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production (/chatops run feature set tribute_autocomplete true)
• Cross post chatops Slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

Rollback Steps

• This feature can be disabled by running the following Chatops command:

/chatops run feature set --project=gitlab-org/gitlab tribute_autocomplete false