v23/model: Remove Platform from the Public API.
v23/model: Remove Platform from the Public API.

There are currently no users of this so we should move this to be internal.

MultiPart: 1/2
Change-Id: I34ae67119762260bec92baa9348794282a571e47

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23