Remove fields from manifest that cause PWA install behavior.

[nstrayer]

This PR removes the fields in web mode that prompt the user to install positron to their desktop. This behavior was caused by positron being setup as a "progressive web app."

Before

After

Potential downsides

I don't think this manifest adjustment will cause any issues with other PWA niceties but it's a possibility. I didn't notice anything out-of-the-ordinary in my brief testing.

QA Notes

I tested locally running the "Positron Server (Web)" debug mode. Should be good for deployed-as-server scenarios as well.

[nstrayer]

Not needed as upstream PR will address.