Remove `simulate_pipeline` feature flag

What does this MR do and why?

Feature flag rollout issue: #364257 (closed)

This replaces the Lint tab with the Validate tab and adds the pipeline simulation feature in the pipeline editor.

The feature flag is already enabled globally on production.

Screenshots or screen recordings

Screen_Recording_2022-07-05_at_19.35.13

How to set up and validate locally

1. Disable the simulate_pipeline feature flag.
2. Follow the steps in !91241 (merged) starting from step 2.

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.
• Update existing test cases to not refer to FF status.
  • https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/368332
  • https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/368333
• Archive old test cases after MR is merged
  • https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/349128
  • https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/349129