UDSRemoteStore: Support the 'root' store parameter

Useful when we're using a daemon with a chroot store, e.g.

  $ NIX_DAEMON_SOCKET_PATH=/tmp/chroot/nix/var/nix/daemon-socket/socket nix-daemon --store /tmp/chroot

Then the client can now connect with

  $ nix build --store unix:///tmp/chroot/nix/var/nix/daemon-socket/socket?root=/tmp/chroot nixpkgs#hello

src/libstore/local-store.hh

@@ -83,9 +83,6 @@ private:
 
 public:
 
-    PathSetting realStoreDir_;
-
-    const Path realStoreDir;
     const Path dbDir;
     const Path linksDir;
     const Path reservedPath;
@@ -279,8 +276,6 @@ private:
     void signPathInfo(ValidPathInfo & info);
     void signRealisation(Realisation &);
 
-    Path getRealStoreDir() override { return realStoreDir; }
-
     void createUser(const std::string & userName, uid_t userId) override;
 
     // XXX: Make a generic `Store` method