qemu-guest: remove security.rngd setting

Since release 20.09 `rngd.enable` defaults to false, so this setting is redundant.

Also fix the `qemu-quest` section of the manual that incorrectly claimed
that `rngd` was enabled.

nixos/modules/profiles/qemu-guest.nix

@@ -1,7 +1,7 @@
 # Common configuration for virtual machines running under QEMU (using
 # virtio).
 
-{ lib, ... }:
+{ ... }:
 
 {
   boot.initrd.availableKernelModules = [ "virtio_net" "virtio_pci" "virtio_mmio" "virtio_blk" "virtio_scsi" "9p" "9pnet_virtio" ];
@@ -14,6 +14,4 @@
       # to the *boot time* of the host).
       hwclock -s
     '';
-
-  security.rngd.enable = lib.mkDefault false;
 }