nixos/filesystems/ext: Don't try to load ext3 module

This module doesn't exist since v4.3, where the ext3 driver was removed
as ext4.ko can mount ext3 filesystems as well.

nixos/modules/tasks/filesystems/ext.nix

···
 
     system.fsPackages = [ pkgs.e2fsprogs ];
 
-    boot.initrd.availableKernelModules = [ "ext2" "ext3" "ext4" ];
+    # As of kernel 4.3, there is no separate ext3 driver (they're also handled by ext4.ko)
+    boot.initrd.availableKernelModules = [ "ext2" "ext4" ];
 
     boot.initrd.extraUtilsCommands =
       ''