pgraph: Remove use of Flags struct in favour of Value API

One small step to completely cleaning up the pgraph package so that we
can eventually fix the code that would otherwise create a cycle!

pgraph/pgraph.go

@@ -42,11 +42,6 @@ const (
 	graphStatePaused
 )
 
-// Flags contains specific constants used by the graph.
-type Flags struct {
-	Debug bool
-}
-
 // Graph is the graph structure in this library.
 // The graph abstract data type (ADT) is defined as follows:
 // * the directed graph arrows point from left to right ( -> )
@@ -60,7 +55,6 @@ type Graph struct {
 	kv        map[string]interface{}        // some values associated with the graph
 
 	// legacy
-	Flags     Flags
 	state     graphState
 	fastPause bool        // used to disable pokes for a fast pause
 	mutex     *sync.Mutex // used when modifying graph State variable
@@ -158,7 +152,6 @@ func (g *Graph) Copy() *Graph {
 		kv:        g.kv,
 
 		// legacy
-		Flags:     g.Flags,
 		state:     g.state,
 		mutex:     g.mutex,
 		wg:        g.wg,