Remove urls content in Spawning response
Currently SplitOptInOutJobRunner job runner  displays following fields:
```
    urls_columns: List[str]
    num_opt_in_urls: int
    num_opt_out_urls: int
    num_urls: int
    num_scanned_rows: int
    has_urls_columns: bool
    opt_in_urls: List[OptUrl]
    opt_out_urls: List[OptUrl]
```

For first phase, `opt_in_urls` and  `opt_out_urls` shouldn't be shown meanwhile we found a better way to store it.
cc. @julien-c @severo 
Remove urls content in Spawning response: Currently SplitOptInOutJobRunner job runner  displays following fields:
```
    urls_columns: List[str]
    num_opt_in_urls: int
    num_opt_out_urls: int
    num_urls: int
    num_scanned_rows: int
    has_urls_columns: bool
    opt_in_urls: List[OptUrl]
    opt_out_urls: List[OptUrl]
```

For first phase, `opt_in_urls` and  `opt_out_urls` shouldn't be shown meanwhile we found a better way to store it.
cc. @julien-c @severo 