Drop the `chunks` field in the client manifest for SSR (#50959)
The `ssrModuleMapping`/`edgeSSRModuleMapping` fields store information
for the Flight client that does SSR to resolve the correct modules in
the SSR bundle. This works as a module ID mapping between the client
bundle and the SSR bundle. During the mapping, we can drop the `chunks`
field because we don't actually have external chunks to load during SSR
as everything is locally `require`'d.
This reduces the client manifest size by at least 20%.
