remove rebootAfterKernelUpgrade option

This is not a feature specific to the mailserver. Indeed, the feature
was added to `system.autoUpgrade.allowReboot` with NixOS 19.09 and it
has better detection if a reboot is necessary.

For the system.autoUpgrade there is no kexec option, but the use was
discouraged.

default.nix

@@ -1228,28 +1228,6 @@ in
 
     };
 
-    rebootAfterKernelUpgrade = {
-      enable = mkOption {
-        type = types.bool;
-        default = false;
-        example = true;
-        description = ''
-          Whether to enable automatic reboot after kernel upgrades.
-          This is to be used in conjunction with `system.autoUpgrade.enable = true;`
-          This can also be achieved via `system.autoUpgrade.allowReboot = true;`
-        '';
-      };
-      method = mkOption {
-        type = types.enum [ "reboot" "systemctl kexec" ];
-        default = "reboot";
-        description = ''
-          Whether to issue a full "reboot" or just a "systemctl kexec"-only reboot.
-          It is recommended to use the default value because the quicker kexec reboot has a number of problems.
-          Also if your server is running in a virtual machine the regular reboot will already be very quick.
-        '';
-      };
-    };
-
     backup = {
       enable = mkEnableOption "backup via rsnapshot";
 
@@ -1317,6 +1295,13 @@ in
     (lib.mkRemovedOptionModule [ "mailserver" "fullTextSearch" "indexAttachments" ] ''
     Text attachments are always indexed since fts-xapian 1.4.8
     '')
+    (lib.mkRenamedOptionModule
+      [ "mailserver" "rebootAfterKernelUpgrade" "enable" ]
+      [ "system" "autoUpgrade" "allowReboot" ]
+    )
+    (lib.mkRemovedOptionModule [ "mailserver" "rebootAfterKernelUpgrade" "method" ] ''
+    Use `system.autoUpgrade` instead.
+    '')
     ./mail-server/assertions.nix
     ./mail-server/borgbackup.nix
     ./mail-server/debug.nix
@@ -1333,6 +1318,5 @@ in
     ./mail-server/rspamd.nix
     ./mail-server/nginx.nix
     ./mail-server/kresd.nix
-    ./mail-server/post-upgrade-check.nix
   ];
 }

mail-server/post-upgrade-check.nix

@@ -1,46 +0,0 @@
-#  nixos-mailserver: a simple mail server
-#  Copyright (C) 2016-2018  Robin Raymond
-#
-#  This program is free software: you can redistribute it and/or modify
-#  it under the terms of the GNU General Public License as published by
-#  the Free Software Foundation, either version 3 of the License, or
-#  (at your option) any later version.
-#
-#  This program is distributed in the hope that it will be useful,
-#  but WITHOUT ANY WARRANTY; without even the implied warranty of
-#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-#  GNU General Public License for more details.
-#
-#  You should have received a copy of the GNU General Public License
-#  along with this program. If not, see <http://www.gnu.org/licenses/>
-
-{ config, pkgs, lib, ... }:
-
-with lib;
-
-let
-  cfg = config.mailserver;
-in
-{
-  config = mkIf (cfg.enable && cfg.rebootAfterKernelUpgrade.enable) {
-    systemd.services.nixos-upgrade.serviceConfig.ExecStartPost = pkgs.writeScript "post-upgrade-check" ''
-      #!${pkgs.stdenv.shell}
-
-      # Checks whether the "current" kernel is different from the booted kernel
-      # and then triggers a reboot so that the "current" kernel will be the booted one.
-      # This is just an educated guess. If the links do not differ the kernels might still be different, according to spacefrogg in #nixos.
-
-      current=$(readlink -f /run/current-system/kernel)
-      booted=$(readlink -f /run/booted-system/kernel)
-
-      if [ "$current" == "$booted" ]; then
-        echo "kernel version seems unchanged, skipping reboot" | systemd-cat --priority 4 --identifier "post-upgrade-check";
-      else
-        echo "kernel path changed, possibly a new version" | systemd-cat --priority 2 --identifier "post-upgrade-check"
-        echo "$booted" | systemd-cat --priority 2 --identifier "post-upgrade-kernel-check"
-        echo "$current" | systemd-cat --priority 2 --identifier "post-upgrade-kernel-check"
-        ${cfg.rebootAfterKernelUpgrade.method}
-      fi
-    '';
-  };
-}