chore(element): drop unsupported parameter 'purge_networks' on 'docker_container' module

roles/element/docs/docker.md

@@ -14,7 +14,6 @@ are available under the `element_container_` prefix:
 - `labels`
 - `networks`
 - `etc_hosts`
-- `purge_networks`
 
 The following variables are pre-populated by the role, so override them with care:
 

roles/element/tasks/deploy-docker.yml

@@ -30,4 +30,3 @@
     networks: "{{ element_container_networks | default(omit) }}"
     etc_hosts: "{{ element_container_etc_hosts | default(omit) }}"
     restart_policy: "{{ element_container_restart_policy }}"
-    purge_networks: "{{ element_container_purge_networks | default(omit) }}"