chromium: Remove google_default_client_{id,secret}

Reason: Google is limiting access to their private Chrome APIs starting
on March 15, 2021 [0]. Closes NixOS#110245.

From the mailing list thread [1]:
"The changes we announced affect the OAuth 2.0 client id and secret
which are used for signing into Chrome, not the API key."
"To avoid using that API, it's sufficient to either not set the OAuth
2.0 credentials, or disabling the Google signin integration" (e.g. by
passing the flag --allow-browser-signin=false)

[0]: https://blog.chromium.org/2021/01/limiting-private-api-availability-in.html
[1]: https://groups.google.com/a/chromium.org/g/chromium-packagers/c/SG6jnsP4pWM/

pkgs/applications/networking/browsers/chromium/common.nix

@@ -250,13 +250,10 @@ let
       symbol_level = 0;
       fieldtrial_testing_like_official_build = true;
 
-      # Google API keys, see:
-      #   http://www.chromium.org/developers/how-tos/api-keys
-      # Note: These are for NixOS/nixpkgs use ONLY. For your own distribution,
-      # please get your own set of keys.
+      # Google API key, see: https://www.chromium.org/developers/how-tos/api-keys
+      # Note: The API key is for NixOS/nixpkgs use ONLY.
+      # For your own distribution, please get your own set of keys.
       google_api_key = "AIzaSyDGi15Zwl11UNe6Y-5XW_upsfyw31qwZPI";
-      google_default_client_id = "404761575300.apps.googleusercontent.com";
-      google_default_client_secret = "9rIFQjfnkykEmqb6FfjJQD1D";
     } // optionalAttrs proprietaryCodecs {
       # enable support for the H.264 codec
       proprietary_codecs = true;