Remove matrix_container_global_registry_prefix variable

This is done for a few reasons:

- less globals and more indepdendence for each role is better. We rely
  on various externally-hosted roles and they don't rely on this global
  either.

- `matrix_container_global_registry_prefix` could make people think they
  could just override this variable and have all their images pull from
  elsewhere. This is rarely the case, unless you've taken special care
  to mirror all the various components (from their respective
  registries) to your own. In such a case, you probably know what you're
  mirroring and can adjust individual variables.

- nowadays, various components live on different registries.
  With Docker Inc tightening rate limits for Docker Hub, it's even more
  likely that we'll see increased diversity in where images are hosted

roles/custom/matrix-user-verification-service/defaults/main.yml

@@ -24,8 +24,8 @@ matrix_user_verification_service_config_env_file: "{{ matrix_user_verification_s
 matrix_user_verification_service_docker_src_files_path: "{{ matrix_user_verification_service_base_path }}/docker-src"
 
 # Docker
-matrix_user_verification_service_docker_image_name_prefix: "{{ matrix_container_global_registry_prefix }}"
-matrix_user_verification_service_docker_image: "{{ matrix_user_verification_service_docker_image_name_prefix }}matrixdotorg/matrix-user-verification-service:{{ matrix_user_verification_service_version }}"
+matrix_user_verification_service_docker_image: "{{ matrix_user_verification_service_docker_image_registry_prefix }}matrixdotorg/matrix-user-verification-service:{{ matrix_user_verification_service_version }}"
+matrix_user_verification_service_docker_image_registry_prefix: docker.io/
 matrix_user_verification_service_docker_image_force_pull: "{{ matrix_user_verification_service_docker_image.endswith(':latest') }}"
 
 # The base container network. It will be auto-created by this role if it doesn't exist already.