[keyserver] Deprecate streamJSON

Summary:
The only usages of this function were removed in D9145. Removing it from the codebase allows us to rip out three NPM packages.

Depends on D10165

Test Plan: Flow

Reviewers: michal, kamil

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10166