nixos/openssh: Drop deprecated locations

The changelog entry should give a good indication why I don't think this
is necessary anymore.

nixos/modules/programs/ssh.nix

@@ -26,7 +26,7 @@ let
       + (if h.publicKey != null then h.publicKey else readFile h.publicKeyFile)
     )) + "\n";
 
-  knownHostsFiles = [ "/etc/ssh/ssh_known_hosts" "/etc/ssh/ssh_known_hosts2" ]
+  knownHostsFiles = [ "/etc/ssh/ssh_known_hosts" ]
     ++ map pkgs.copyPathToStore cfg.knownHostsFiles;
 
 in
@@ -232,9 +232,8 @@ in
         description = lib.mdDoc ''
           Files containing SSH host keys to set as global known hosts.
           `/etc/ssh/ssh_known_hosts` (which is
-          generated by {option}`programs.ssh.knownHosts`) and
-          `/etc/ssh/ssh_known_hosts2` are always
-          included.
+          generated by {option}`programs.ssh.knownHosts`) is
+          always included.
         '';
         example = literalExpression ''
           [