Remove old implementation of the Environment folder page

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What does this MR do and why?

Remove old implementation of the Environment folder page.

• Removed environments_folder_new_look feature flag.
• Removed Vue components related to the old environments folder page implementation.
• Removed related spec files.
• Cleaned up app/assets/stylesheets/page_bundles/environments.scss, app/assets/javascripts/environments/mixins/environments_mixin.js, app/assets/javascripts/environments/folder/environments_folder_bundle.js
• Removed code related to the old REST implementation

Screenshots or screen recordings

Removed view	New view

How to set up and validate locally

1. Navigate to any project.
2. In the .gitlab-ci.yml file create environments using a folder name format.
3. In the sidebar navigate to Operate -> Environments.
4. Click on a folder with nested environments.
5. Review the environments folder page.

Related to #452179 (closed)