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
   ];
 }