[appplatform] remove code after `2019-05-01-preview` retired

[msyyc]

After Azure/azure-rest-api-specs#17506 is merged, need to delete related SDK code :

https://github.com/Azure/azure-sdk-for-python/tree/main/sdk/appplatform/azure-mgmt-appplatform/azure/mgmt/appplatform/v2019_05_01_preview

[msyyc]

just need to change the configuration like the following image so that we needn't delete sdk code manualy in the future: