Remove unused StorageItems from browser extensions

Created by: felixfbecker

There are a lot of StorageItems that are not written to anywhere in the code base (they are read though).

https://github.com/sourcegraph/sourcegraph/blob/68b73cac8a1fd9a9f2ad221ee12d53bd53b84d15/client/browser/src/browser/types.ts#L55-L79

@KattMingMing @sqs can we remove them or are they set through GCloud Admin and used by customers? If they are, which of them, and who did we document them to?