Remove SMTP_* env var functionality from docker conf (#5596)

Removes any `SMTP_*` docker container environment variables from having any effect on the default config.

Fixes https://github.com/matrix-org/synapse/issues/5430

docker/conf/homeserver.yaml

@@ -207,22 +207,3 @@ perspectives:
 
 password_config:
    enabled: true
-
-{% if SYNAPSE_SMTP_HOST %}
-email:
-   enable_notifs: false
-   smtp_host: "{{ SYNAPSE_SMTP_HOST }}"
-   smtp_port: {{ SYNAPSE_SMTP_PORT or "25" }}
-   smtp_user: "{{ SYNAPSE_SMTP_USER }}"
-   smtp_pass: "{{ SYNAPSE_SMTP_PASSWORD }}"
-   require_transport_security: False
-   notif_from: "{{ SYNAPSE_SMTP_FROM or "hostmaster@" + SYNAPSE_SERVER_NAME }}"
-   app_name: Matrix
-   # if template_dir is unset, uses the example templates that are part of
-   # the Synapse distribution.
-   #template_dir: res/templates
-   notif_template_html: notif_mail.html
-   notif_template_text: notif_mail.txt
-   notif_for_new_users: True
-   riot_base_url: "https://{{ SYNAPSE_SERVER_NAME }}"
-{% endif %}