hurd: cleanup unmaintained target

This has been not touched in 6 years. Let's remove it to cause less
problems when adding new cross-compiling infrastructure.
This also simplify gcc significantly.

lib/systems/inspect.nix

@@ -27,14 +27,13 @@ rec {
 
     isBSD          = { kernel = { families = { inherit (kernelFamilies) bsd; }; }; };
     isDarwin       = { kernel = { families = { inherit (kernelFamilies) darwin; }; }; };
-    isUnix         = [ isBSD isDarwin isLinux isSunOS isHurd isCygwin ];
+    isUnix         = [ isBSD isDarwin isLinux isSunOS isCygwin ];
 
     isMacOS        = { kernel = kernels.macos; };
     isiOS          = { kernel = kernels.ios; };
     isLinux        = { kernel = kernels.linux; };
     isSunOS        = { kernel = kernels.solaris; };
     isFreeBSD      = { kernel = kernels.freebsd; };
-    isHurd         = { kernel = kernels.hurd; };
     isNetBSD       = { kernel = kernels.netbsd; };
     isOpenBSD      = { kernel = kernels.openbsd; };
     isWindows      = { kernel = kernels.windows; };