Delete feature flag create_project_subscription_graphql_endpoint

What does this MR do and why?

Feature Flag Issue: #429339 (closed)

After successfully rolling out the feature, we need to remove the relevant flag.

Delete feature flag create_project_subscription_graphql_endpoint

Changelog: changed

How to set up and validate locally

1. Checkout the branch and start the application
2. Make sure that you are using your EE license
3. Open the GraphQL explorer
4. Ping the mutation

mutation {
  createProjectSubscription(input: {
    projectPath: "toolbox/gitlab-smoke-tests",
    upstreamPath: "Commit451/test-3-pub"
  } ) {
    project {
      name
    }
    errors
  }
}

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.