Use systemd-boot instead of gummiboot

Gummiboot is part of systemd now so we may as well use it.

nixos/modules/system/boot/loader/efi.nix

@@ -4,19 +4,16 @@ with lib;
 
 {
   options.boot.loader.efi = {
+
     canTouchEfiVariables = mkOption {
       default = false;
-
       type = types.bool;
-
-      description = "Whether or not the installation process should modify efi boot variables.";
+      description = "Whether the installation process is allowed to modify EFI boot variables.";
     };
 
     efiSysMountPoint = mkOption {
       default = "/boot";
-
       type = types.str;
-
       description = "Where the EFI System Partition is mounted.";
     };
   };