Revert "firefox: set build timeout at eight hours"

The default timeout is 10h on Hydra currently, so this timeout
setting is pointless or counterproductive.
This commit seems to have been made in error
https://github.com/NixOS/nixpkgs/pull/96767#issuecomment-724104413

This reverts commit c57fedb14b27292f6433287740e2eec303363ec3.

pkgs/applications/networking/browsers/firefox/packages.nix

@@ -26,7 +26,6 @@ rec {
       broken = stdenv.buildPlatform.is32bit; # since Firefox 60, build on 32-bit platforms fails with "out of memory".
                                              # not in `badPlatforms` because cross-compilation on 64-bit machine might work.
       license = lib.licenses.mpl20;
-      timeout = 28800; # eight hours
     };
     updateScript = callPackage ./update.nix {
       attrPath = "firefox-unwrapped";