Revert "builtins.readFile: Put the references of the file, not those needed to realize the file, into the context"

Reverting commit 451c223dee for now
because it breaks http://hydra.nixos.org/build/46805136, not clear
why.

src/libexpr/primops.cc

@@ -779,9 +779,6 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
     string s = readFile(state.checkSourcePath(path));
     if (s.find((char) 0) != string::npos)
         throw Error(format("the contents of the file ‘%1%’ cannot be represented as a Nix string") % path);
-    context = state.store->isInStore(path) ?
-        state.store->queryPathInfo(state.store->toStorePath(path))->references :
-        PathSet{};
     mkString(v, s.c_str(), context);
 }