Remove feature flag

What does this MR do and why?

This MR removes the gitlab_for_slack_app_instance_and_group_level feature flag which was default enabled.

• #391526 (closed)
• gitlab-com/gl-infra/production#17410 (moved)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Set up a GitLab for Slack app in your instance admin https://docs.gitlab.com/ee/administration/settings/slack_app.html. You will need a Slack Workspace that you are an admin of (you can sign up for your own Slack workspace for free).
2. On both group and project level, you can view the GitLab for Slack app integration (Settings > Integrations > GitLab for Slack app). You should be able to enable the application at group and project levels.

Related to #391526 (closed)