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

changelog.d/5596.bugfix

+Removed the `SYNAPSE_SMTP_*` docker container environment variables. Using these environment variables prevented the docker container from starting in Synapse v1.0, even though they didn't actually allow any functionality anyway. Users are advised to remove `SYNAPSE_SMTP_HOST`, `SYNAPSE_SMTP_PORT`, `SYNAPSE_SMTP_USER`, `SYNAPSE_SMTP_PASSWORD` and `SYNAPSE_SMTP_FROM` environment variables from their docker run commands.

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 %}