This probably never even worked anyway and was a leftover copy/paste
from some other role.

The docs (`docs/configuring-playbook-bridge-mautrix-wsproxy.md`) only
talk about `matrix_mautrix_wsproxy_hostname`, which was only used via
Traefik labels. The endpoint exposed via `matrix-nginx-proxy` (`/_matrix/wsproxy`)
hasn't been mentioned anywhere.