lib: simplify the readonly properties of icu

Call Object.defineProperty() twice to set readonly property is
unnecessary.

PR-URL: https://github.com/nodejs/node/pull/13221


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>