Remove Welsh feature flag

[thomasiles]

The Welsh feature flag is no longer required now the feature has been released.

This commit removes the terraform variable welsh_enabled and environment variable which controlled the settings in the rails app.

See the PR to remove the flag in the Admin.

What problem does this pull request solve?

Trello card: https://trello.com/c/gI4COCYo/2863-remove-welsh-feature-flag

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Has all relevant documentation been updated?

Reminders

If you've made changes to the deployer role (files in modules/deployer-access):

• Remember to run make <environment> forms/account apply on the relevant environments (dev, staging, user-research, and/or prod)
• Check the #govuk-forms-deployment-notifications Slack channel to ensure the apply-forms-terraform-<environment> pipelines have run successfully

[DavidBiddle]

What you've done here looks good but you've got some merge conflicts - shout if you need a re-review once those are resolved