linux: remove xen_dom0 feature entirely

Xen is now enabled unconditionally on kernels that support it, so the
xen_dom0 feature doesn't do anything.  The isXen attribute will now
produce a deprecation warning and unconditionally return true.
Passing in a custom value for isXen is no longer supported.

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

@@ -46,7 +46,6 @@
                        stdenv.hostPlatform != stdenv.buildPlatform
 , extraMeta ? {}
 
-, isXen      ? features.xen_dom0 or false
 , isZen      ? false
 , isLibre    ? false
 , isHardened ? false
@@ -74,7 +73,6 @@ let
     needsCifsUtils = true;
     netfilterRPFilter = true;
     grsecurity = false;
-    xen_dom0 = false;
     ia32Emulation = true;
   } // features) kernelPatches;
 
@@ -178,7 +176,8 @@ let
 
   passthru = {
     features = kernelFeatures;
-    inherit commonStructuredConfig isXen isZen isHardened isLibre;
+    inherit commonStructuredConfig isZen isHardened isLibre;
+    isXen = lib.warn "The isXen attribute is deprecated. All Nixpkgs kernels that support it now have Xen enabled." true;
     kernelOlder = lib.versionOlder version;
     kernelAtLeast = lib.versionAtLeast version;
     passthru = kernel.passthru // (removeAttrs passthru [ "passthru" ]);