linux.configfile: remove unused kernelTarget attr

As far as I can tell this is unused since f95d214cfd.

Fixes: f95d214cfd ("Implement generic kernel build via manual-config")
Fixes: https://github.com/NixOS/nixpkgs/issues/234084

pkgs/os-specific/linux/kernel/generic.nix

@@ -131,8 +131,6 @@ let
     platformName = stdenv.hostPlatform.linux-kernel.name;
     # e.g. "defconfig"
     kernelBaseConfig = if defconfig != null then defconfig else stdenv.hostPlatform.linux-kernel.baseConfig;
-    # e.g. "bzImage"
-    kernelTarget = stdenv.hostPlatform.linux-kernel.target;
 
     makeFlags = lib.optionals (stdenv.hostPlatform.linux-kernel ? makeFlags) stdenv.hostPlatform.linux-kernel.makeFlags
       ++ extraMakeFlags;