Revert part of #9982 to be in line with #9925

When creating PR #9982, I undid a line of PR #9925, that was some
cleanups and fixes, so this undoes that damage.

nixos/modules/tasks/encrypted-devices.nix

···
       label = mkOption {
         default = null;
         example = "rootfs";
-        type = types.uniq (types.nullOr types.str);
+        type = types.nullOr types.str;
         description = "Label of the unlocked encrypted device. Set <literal>fileSystems.&lt;name?&gt;.device</literal> to <literal>/dev/mapper/&lt;label&gt;</literal> to mount the unlocked device.";
       };