Clean up `pipeline_editor_file_tree` feature flag

What does this MR do and why?

Feature flag cleanup for #357219 (closed).

By default, the file tree will now be available on the pipeline editor.

Screenshots or screen recordings

No visual changes; everything should work as expected.

Screen_Recording_2022-05-16_at_14.51.52

How to set up and validate locally

1. Disable the feature flag.

   Feature.disable(:pipeline_editor_file_tree)

2. Visit the pipeline editor. CI/CD > Editor
3. The file tree button should still show up next to the branch switcher. If your config file uses external YAML configurations through the include keyword, they will show up in the file tree. Otherwise, the file tree will show a tip with a link to the documentation.

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.