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>

wgengine/netstack/netstack_test.go

@@ -45,7 +45,7 @@ func TestInjectInboundLeak(t *testing.T) {
 			t.Logf(format, args...)
 		}
 	}
-	sys := tsd.NewSystemWithEventBus()
+	sys := tsd.NewSystem()
 	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
 		Tun:           tunDev,
 		Dialer:        dialer,
@@ -101,7 +101,7 @@ func getMemStats() (ms runtime.MemStats) {
 
 func makeNetstack(tb testing.TB, config func(*Impl)) *Impl {
 	tunDev := tstun.NewFake()
-	sys := tsd.NewSystemWithEventBus()
+	sys := tsd.NewSystem()
 	sys.Set(new(mem.Store))
 	dialer := new(tsdial.Dialer)
 	logf := tstest.WhileTestRunningLogger(tb)