use Linux kernel sources associated with openwrt by default

devices/families/qemu.nix

@@ -5,11 +5,6 @@
   ];
   config = {
     kernel = {
-      src = pkgs.pkgsBuildBuild.fetchurl {
-        name = "linux.tar.gz";
-        url = "https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.137.tar.gz";
-        hash = "sha256-PkdzUKZ0IpBiWe/RS70J76JKnBFzRblWcKlaIFNxnHQ=";
-      };
       config = {
         MTD = "y";
         MTD_BLOCK = "y";