Remove Slack notifications as an option from the integrations list

What does this MR do and why?

This MR targets the gitlab.com users that have never setup Slack notifications. As per #376134 (closed), for this scenario, we should:

For gitlab.com users that have never setup Slack notifications:

• Remove Slack notifications as an option from the integrations list

Technically, this solution doesn't account for gitlab.com-only users. However, considering the flag will be flipped only on gitlab.com this allows to ship this solution as that matching the requirement. The self-hosted instances will not get this automatically enabled and, hence, won't experience any changes even after the flag is flipped on gitlab.com

Screenshots or screen recordings

	Flag is ON	Flag is OFF
Slack notifications is not active
Slack notifications is active

How to set up and validate locally

1. Visit project's integrations, like http://127.0.0.1:3000/flightjs/Flight/-/settings/integrations.

2. If you have never set up Slack notifications on the instance, you will find it in the Add an integration table

3. Enable the Feature Flag

   Feature.enable(:integration_slack_app_notifications)

4. Reload the page. The Slack notifications should be gone from the table

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383364 (closed)