nixos/systemd: remove mymachines nss module from passwd: and group: lines

From the systemd release notes:

nss-mymachines lost support for resolution of users and groups, and now
only does resolution of hostnames. This functionality is now provided by
nss-systemd. Thus, the 'mymachines' entry should be removed from the
'passwd:' and 'group:' lines in /etc/nsswitch.conf (and 'systemd' added
if it is not already there).

nixos/modules/system/boot/systemd.nix

@@ -907,11 +907,9 @@ in
       )
       ]);
       passwd = (mkMerge [
-        [ "mymachines" ]
         (mkAfter [ "systemd" ])
       ]);
       group = (mkMerge [
-        [ "mymachines" ]
         (mkAfter [ "systemd" ])
       ]);
     };