nixos/firmware: remove restrictive hardware.enableAllFirmware assertion

Remove the too restrictive hardware.enableAllFirmware assertion
introduced in commit 05aa80c06a ("hardware: add
enableRedistributalFirmware").

This assertion is too restrictive because it enforces globally enabling
unfree packages without allowing explicit whitelisting:

     hardware.enableAllFirmware = true;

    -nixpkgs.config.allowUnfree = true;
    +nixpkgs.config.allowUnfreePredicate = pkg:
    +    builtins.elem (lib.getName pkg) [
    +      "b43-firmware"
    +      "broadcom-bt-firmware"
    +      "facetimehd-calibration"
    +      "facetimehd-firmware"
    +      "xow_dongle-firmware"
    +    ];

Declaring neither nixpkgs.config.allowUnfree nor
nixpkgs.config.allowUnfreePredicate without this
hardware.enableAllFirmware assertion results in detailed and instructive
evaluation error messages.

nixos/modules/hardware/all-firmware.nix

@@ -85,16 +85,6 @@ in
         ++ lib.optional pkgs.stdenv.hostPlatform.isAarch raspberrypiWirelessFirmware;
     })
     (lib.mkIf cfg.enableAllFirmware {
-      assertions = [
-        {
-          assertion = cfg.enableAllFirmware -> pkgs.config.allowUnfree;
-          message = ''
-            the list of hardware.enableAllFirmware contains non-redistributable licensed firmware files.
-              This requires nixpkgs.config.allowUnfree to be true.
-              An alternative is to use the hardware.enableRedistributableFirmware option.
-          '';
-        }
-      ];
       hardware.firmware =
         with pkgs;
         [