The variable was necessary when multiple playbooks could have
potentially tried to manage a shared `devture-traefik.serivce` systemd service
and shared `/devture-traefik` directory.

Since adcc6d9723086f65f1a72, we use our own `/matrix/traefik`
(`matrix-traefik.service`) installation and no conflicts can arise.
It's safe to always enable the role, just like we do with all the other roles.

The migration is automatic. Existing users should experience a bit of
downtime until the playbook runs to completion, but don't need to do
anything manually.

This change is provoked by https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2535

While my statements there ("Traefik is a shared component among
sibling/related playbooks and should retain its global
non-matrix-prefixed name and path") do make sense, there's another point
of view as well.

With the addition of docker-socket-proxy support in bf2b54080789f7e,
we potentially introduced another non-`matrix-`-prefixed systemd service
and global path (`/devture-container-socket-proxy`). It would have
started to become messy.

Traefik always being called `devture-traefik.service` and using the `/devture-traefik` path
has the following downsides:

- different playbooks may write to the same place, unintentionally,
  before you disable the Traefik role in some of them.
  If each playbook manages its own installation, no such conflicts
  arise and you'll learn about the conflict when one of them starts its
  Traefik service and fails because the ports are already in use

- the data is scattered - backing up `/matrix` is no longer enough when
  some stuff lives in `/devture-traefik` or `/devture-container-socket-proxy` as well;
  similarly, deleting `/matrix` is no longer enough to clean up

For this reason, the Traefik instance managed by this playbook
will now be called `matrix-traefik` and live under `/matrix/traefik`.

This also makes it obvious to users running multiple playbooks, which
Traefik instance (powered by which playbook) is the active one.
Previously, you'd look at `devture-traefik.service` and wonder which
role was managing it.