fix: remove namespace prefix since environment_slug is already taking it into account

The above configuration will deploy 2 enviroments on deployments jobs (`tf-plan-*`, `tf-review`, `tf-destroy-*`, etc.).

:info: When using both this feature and [GitLab backend](#gitlab-managed-terraform-state-default), the state name will be `<namespace>_<environment_slug>`

where `namespace` is built from `TF_ENVIRONMENT_NAMESPACE` (stripped of punctuation characters and converted to lowercase).

> [!note]

> When using both this feature and [GitLab backend](#gitlab-managed-terraform-state-default),

> the state name (based on `$CI_ENVIRONMENT_SLUG`) will implicitly contain the environment namespace.

> As examples, with the above namespace, it will respectively be for the various environments:

> - For 'some-project-id': `review-some-proje-k6x51d`, `integration-some-55v14l`, `staging-some-proj-p2bkuw`, `production-some-p-enhwzj`

> - For 'some-other-project-id': `review-some-other-8uad93`, `integration-some-ozug25`, `staging-some-othe-2pelfe`, `production-some-o-og55ux`

>

> If needed, the whole HTTP state URL can be overridden with `TF_ADDRESS` (defaults to `${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/terraform/state/${environment_slug}`) environment variable.

### Supported output artifacts

      fi

      # If TF_ADDRESS is unset then default to GitLab backend in current project

      prefix="${environment_namespace:+${environment_namespace}_}"

      TF_ADDRESS="${TF_ADDRESS:-${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/terraform/state/${prefix}${environment_slug}}"

      # environment slug implicitly contains the environment namespace (matrix support)

      TF_ADDRESS="${TF_ADDRESS:-${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/terraform/state/${environment_slug}}"

      # Set variables for the HTTP backend to default to TF_* values

      # see: https://developer.hashicorp.com/terraform/language/settings/backends/http