bspwm/unstable.nix: clean up unused file

06c403e846dca7c06924a1cf754cfa10b608616f removed the bspwm-unstable
attribute, but forgot to remove the file which has been done in
this commit.

pkgs/applications/window-managers/bspwm/unstable.nix

@@ -1,31 +0,0 @@
-{ stdenv, fetchFromGitHub, libxcb, libXinerama, xcbutil, xcbutilkeysyms, xcbutilwm }:
-
-stdenv.mkDerivation {
-  name = "bspwm-unstable-2016-09-30";
-
-
-  src = fetchFromGitHub {
-    owner   = "baskerville";
-    repo    = "bspwm";
-    rev     = "8664c007e44de162c1597fd7e163635b274fb747";
-    sha256  = "0clvpz32z38i8kr10hqlifa661szpfn93c63m2aq2h4dwmr44slz";
-  };
-
-  buildInputs = [ libxcb libXinerama xcbutil xcbutilkeysyms xcbutilwm ];
-
-  buildPhase = ''
-    make PREFIX=$out
-  '';
-
-  installPhase = ''
-    make PREFIX=$out install
-  '';
-
-  meta = {
-    description = "A tiling window manager based on binary space partitioning (git version)";
-    homepage = "https://github.com/baskerville/bspwm";
-    maintainers = [ lib.maintainers.meisternu lib.maintainers.epitrochoid ];
-    license = lib.licenses.bsd2;
-    platforms = lib.platforms.linux;
-  };
-}