gokrazy/tsapp: remove implicit heartbeat package

The heartbeat package does nothing if not configured anyway, so don't
even put it in the image and pay the cost of it running.

Updates #13038
Updates #1866

Change-Id: Id22c0fb1f8395ad21ab0e0350973d31730e8d39f

gokrazy/tsapp/config.json

@@ -4,6 +4,11 @@
         "NoPassword": true
     },
     "SerialConsole": "ttyS0,115200",
+    "GokrazyPackages": [
+        "github.com/gokrazy/gokrazy/cmd/dhcp",
+        "github.com/gokrazy/gokrazy/cmd/randomd",
+        "github.com/gokrazy/gokrazy/cmd/ntp"
+    ],
     "Packages": [
         "github.com/gokrazy/serial-busybox",
         "github.com/gokrazy/breakglass",