[clean up] Remove old implementation of Environment folder page

Description

Now when the new folders page is online, we should clean up unused code.

Related files

a good starting point is the following files list:

app/assets/javascripts/environments/folder/environments_folder_view.vue
app/assets/javascripts/environments/folder/environments_folder_bundle.js
app/assets/javascripts/environments/mixins/environments_mixin.js
app/assets/javascripts/environments/components/container.vue
app/assets/javascripts/environments/folder/environments_folder_app.vue

We should not forget the related spec files and feature flags.

One of the next steps could be removing non-graphql related functionality from the environment item component.