Remove public export for unstable-shared-subset.js (#23261)

The unstable-shared-subset.js file is not a public module — it's a
private module that the "react" package maps to when it's accessed from
the "react-server" package.

We originally added it because it was required to make our Rollup
configuration work, because at the time only "public" modules could act
as the entry point for a build artifact — that's why it's prefixed with
"unstable". We've since updated our Rollup config to support private
entry points, so we can remove the extra indirection.