all: update the tsd.System constructor name (#15372)

Replace NewSystemWithEventBus with plain NewSystem, and update all usage.
See https://github.com/tailscale/tailscale/pull/15355#discussion_r2003910766

Updates #15160

Change-Id: I64d337f09576b41d9ad78eba301a74b9a9d6ebf4
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -341,7 +341,7 @@ func run() (err error) {
 
 	// Install an event bus as early as possible, so that it's
 	// available universally when setting up everything else.
-	sys := tsd.NewSystemWithEventBus()
+	sys := tsd.NewSystem()
 
 	// Parse config, if specified, to fail early if it's invalid.
 	var conf *conffile.Config

cmd/tailscaled/tailscaled_windows.go

@@ -327,7 +327,7 @@ func beWindowsSubprocess() bool {
 		log.Printf("Error pre-loading \"%s\": %v", fqWintunPath, err)
 	}
 
-	sys := tsd.NewSystemWithEventBus()
+	sys := tsd.NewSystem()
 	netMon, err := netmon.New(log.Printf)
 	if err != nil {
 		log.Fatalf("Could not create netMon: %v", err)