refactor!: remove KEYCLOAK2* env vars

The experiment is over.

homeserver.yaml.tmpl

@@ -604,20 +604,6 @@ oidc_providers:
         display_name_template: "{{ "{{ user.name }}" }}"
   {{ end }}
 
-  {{ if eq (env "KEYCLOAK2_ENABLED") "1" }}
-  - idp_id: {{ env "KEYCLOAK2_ID" }}
-    idp_name: {{ env "KEYCLOAK2_NAME" }}
-    issuer: "{{ env "KEYCLOAK2_URL" }}"
-    client_id: "{{ env "KEYCLOAK2_CLIENT_ID" }}"
-    client_secret: "{{ secret "keycloak2_client_secret" }}"
-    scopes: ["openid", "profile"]
-    allow_existing_users: {{ env "KEYCLOAK2_ALLOW_EXISTING_USERS" }}
-    user_mapping_provider:
-      config:
-        localpart_template: "{{ "{{ user.preferred_username }}" }}"
-        display_name_template: "{{ "{{ user.name }}" }}"
-  {{ end }}
-
 # Additional settings to use with single-sign on systems such as OpenID Connect,
 # SAML2 and CAS.
 #