nixos/modules: fix systemd start rate-limits

These were broken since 2016:
https://github.com/systemd/systemd/commit/f0367da7d1a61ad698a55d17b5c28ddce0dc265a
since StartLimitIntervalSec got moved into [Unit] from [Service].
StartLimitBurst has also been moved accordingly, so let's fix that one
too.

NixOS systems have been producing logs such as:
/nix/store/wf98r55aszi1bkmln1lvdbp7znsfr70i-unit-caddy.service/caddy.service:31:
Unknown key name 'StartLimitIntervalSec' in section 'Service', ignoring.

I have also removed some unnecessary duplication in units disabling
rate limiting since setting either interval or burst to zero disables it
(https://github.com/systemd/systemd/blob/ad16158c10dfc3258831a9ff2f1a988214f51653/src/basic/ratelimit.c#L16)

nixos/modules/services/networking/namecoind.nix

@@ -165,6 +165,8 @@ in
       after    = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
 
+      startLimitIntervalSec = 120;
+      startLimitBurst = 5;
       serviceConfig = {
         User  = "namecoin";
         Group = "namecoin";
@@ -176,8 +178,6 @@ in
         TimeoutStopSec     = "60s";
         TimeoutStartSec    = "2s";
         Restart            = "always";
-        StartLimitInterval = "120s";
-        StartLimitBurst    = "5";
       };
 
       preStart = optionalString (cfg.wallet != "${dataDir}/wallet.dat")  ''