[Feature flag] Clean up `environment_settings_to_graphql`

What does this MR do and why?

The environment_settings_to_graphql FF was enabled by default in %16.1. With this MR we are removing the old code for the disabled state and leaving the new code.

Screenshots or screen recordings

No visual changes, as the FF was already enabled by default. The related code migrated the environments settings page from axios to graphql and introduced a new field to select the cluster agent.

New environment	Edit environment

How to set up and validate locally

1. Go to the Environments page
2. Create a new environment / update an existing environment
3. Verify the settings work as expected
4. Verify the agent selector is available on the page.

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.

Review progress:

• backend developer review
• backend maintainer review
• frontend developer review
• frontend maintainer review
• documentation review

Related to #412332 (closed)