linux_beagleboard: remove

The package is out-of-date and has no maintainer.
It should be now possible to just mainline kernel.
Support for that could be added by copying the right dtb file in our linux_rpi kernel.
I do not have the hardware to test this.

cc @dezgeg @dhess

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

@@ -1,34 +0,0 @@
-{ stdenv, buildPackages, fetchFromGitHub, perl, buildLinux, ubootTools, dtc, ... } @ args:
-
-let
-  modDirVersion = "4.14.12";
-  tag = "r23";
-in
-stdenv.lib.overrideDerivation (buildLinux (args // rec {
-  version = "${modDirVersion}-ti-${tag}";
-  inherit modDirVersion;
-
-  src = fetchFromGitHub {
-    owner = "beagleboard";
-    repo = "linux";
-    rev = "${version}";
-    sha256 = "07hdv2h12gsgafxsqqr7b0fir10rv9k66riklpjba2cg6x0p2nr4";
-  };
-
-  kernelPatches = args.kernelPatches;
-
-  features = {
-    efiBootStub = false;
-  } // (args.features or {});
-
-  extraMeta.hydraPlatforms = [ "armv7l-linux" ];
-} // (args.argsOverride or {}))) (oldAttrs: {
-
-  # This kernel will run mkuboot.sh.
-  postPatch = ''
-    patchShebangs scripts/
-  '';
-
-  nativeBuildInputs = oldAttrs.nativeBuildInputs ++ [ dtc ubootTools ];
-
-})