Remove assertion about having the SECRET_KEY set

* Invenio-Config already takes care of checking the value in real
  Invenio applications and issues a warning if it's unset

invenio_config_tuw/startup/config.py

@@ -237,9 +237,6 @@ def assemble_keycloak_config_from_parts(app):
 def populate_unset_salt_values(app):
     """Populate the salt values if they're not set yet."""
     secret_key = app.config.get("SECRET_KEY", None)
-    if secret_key is None:
-        raise RuntimeError("SECRET_KEY configuration is unset! Aborting.")
-
     app.config.setdefault("CSRF_SECRET_SALT", secret_key)
     app.config.setdefault("SECURITY_RESET_SALT", secret_key)
     app.config.setdefault("SECURITY_LOGIN_SALT", secret_key)