--trusted-testers now unnecessary? (errors in 4.0.1)

Hi there, and a quick bug report.

I've just upgraded to chrome-webstore-upload-cli `v4.0.1` from `v3.5.0`. I was using the `--trusted-testers` flag, but publishing with this shows error:
```
Invalid value at 'publish_type' (type.googleapis.com/google.chrome.webstore.v2.PublishType), "TRUSTED_TESTERS"
```

I haven't been following along with changes to the webstore api, but "trusted testers" seems to be managed by setting an extension private, and specifying a list of trusted testers in the webstore settings ui.

My "fix" was to remove `--trusted-testers` flag, and all seems to work. So maybe the associated logic can be removed?

Many thanks for your work on chrome-webstore-upload-cli - it's very useful.

Start at the CLI entry point that defines the --trusted-testers flag and follow how it maps to the Chrome Web Store publish request. Confirm the v4.0.1 error, remove or disable the obsolete publish value, and verify that publishing without the flag still works.

Written by the indexing model from the issue text.