browser.secureStorage: Add a way to remove items from secureStorage

[oliverdunk]

There are a few things that make the secureStorage proposal feel less real, and one of those is that there currently isn't a way to delete data. We should add a function for that. We could also consider a way of getting the available storage quota left as suggested in #154 (comment).

[oliverdunk]

Closing this. I am still personally interested in the API but there hasn't been any recent movement on implementation. With that in mind, it doesn't feel worthwhile updating the proposal.

From Chrome's side, we see the use case for the idea at a very high level but are worried about the number of implementations for different platforms that would be needed (using macOS / Windows Hello APIs etc.) and the long-term support costs even after this was implemented across all platforms.