delete remnant 'required': false (#1955)

api/core/model_runtime/model_providers/togetherai/togetherai.yaml

@@ -30,7 +30,6 @@ model_credential_schema:
     label:
       en_US: API Key
     type: secret-input
-    required: false
     placeholder:
       zh_Hans: 在此输入您的 API Key
       en_US: Enter your API Key