wgengine: remove SetNetInfoCallback method from Engine

LocalBackend can talk to magicsock on its own to do this without
the "Engine" being involved.

(Continuing a little side quest of cleaning up the Engine
interface...)

Updates #cleanup

Change-Id: I8654acdca2b883b1bd557fdc0cfb90cd3a418a62
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/wgengine.go

@@ -35,9 +35,6 @@ type Status struct {
 // Exactly one of Status or error is non-nil.
 type StatusCallback func(*Status, error)
 
-// NetInfoCallback is the type used by Engine.SetNetInfoCallback.
-type NetInfoCallback func(*tailcfg.NetInfo)
-
 // NetworkMapCallback is the type used by callbacks that hook
 // into network map updates.
 type NetworkMapCallback func(*netmap.NetworkMap)
@@ -125,10 +122,6 @@ type Engine interface {
 	// The network map should only be read from.
 	SetNetworkMap(*netmap.NetworkMap)
 
-	// SetNetInfoCallback sets the function to call when a
-	// new NetInfo summary is available.
-	SetNetInfoCallback(NetInfoCallback)
-
 	// DiscoPublicKey gets the public key used for path discovery
 	// messages.
 	DiscoPublicKey() key.DiscoPublic