use Linux kernel sources associated with openwrt by default

devices/belkin-rt3200/default.nix

@@ -76,11 +76,6 @@
       ];
       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=";
-      };
       extraPatchPhase = ''
           ${pkgs.openwrt.applyPatches.mediatek}
       '';