chromium: remove enableNaCl option

This hasn't worked since 2016 (https://github.com/NixOS/nixpkgs/issues/13983)
and is being removed very soon, per "Q4 2019" in
https://developer.chrome.com/native-client/migration

(It's staying around for "Chrome Apps", but those only run on ChromeOS now.)

earth.google.com users can use https://earth.google.com/web/?beta=1 to get
the wasm version instead of the PNaCl version.