feat: update signout functionality to use OpenID configuration for logout URL and remove the logout variable from config

backend/open_webui/config.py

@@ -384,12 +384,6 @@ OAUTH_PROVIDER_NAME = PersistentConfig(
     os.environ.get("OAUTH_PROVIDER_NAME", "SSO"),
 )
 
-OAUTH_LOGOUT_URL = PersistentConfig(
-    "OAUTH_LOGOUT_URL",
-    "oauth.oidc.logout_url",
-    os.environ.get("OAUTH_LOGOUT_URL", ""),
-)
-
 OAUTH_USERNAME_CLAIM = PersistentConfig(
     "OAUTH_USERNAME_CLAIM",
     "oauth.oidc.username_claim",