net/netmon: remove usage of direct callbacks from netmon (#17292)

The callback itself is not removed as it is used in other repos, making
it simpler for those to slowly transition to the eventbus.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -1668,6 +1668,7 @@ func newLocalBackendWithMockEngineAndControl(t *testing.T, enableLogging bool) (
 	sys := tsd.NewSystemWithBus(bus)
 	sys.Set(dialer)
 	sys.Set(dialer.NetMon())
+	dialer.SetBus(bus)
 
 	magicConn, err := magicsock.NewConn(magicsock.Options{
 		Logf:              logf,