handle not found response when deleting app in wal rollback

[fairclothjm]

During the revoke call, Vault prints the below warning in operational logs:

secrets.azure.azure_1a4bd492: rollback error deleting App: err="autorest#WithErrorUnlessStatusCode: DELETE https://graph.microsoft.com/v1.0/directory/deletedItems/4ef6b566XXXXXXXXXa-1aa3a778abb7 failed with 404 Not Found: StatusCode=404"
secrets.azure.azure_1a4bd492: rollback error deleting App: err="autorest#WithErrorUnlessStatusCode: DELETE https://graph.microsoft.com/v1.0/directory/deletedItems/68f3cXXXXXXf-9b8c-7af3eef58b06 failed with 404 Not Found: StatusCode=404"

Previously, the delete call didn’t return 404 if the app isn’t found. However, we recently switched the underlying API.