Remove commentsEnabled AP extension

This isn't used.

activitystreams/activity.go

@@ -11,10 +11,7 @@ const (
 	toPublic  = "https://www.w3.org/ns/activitystreams#Public"
 )
 
-var Extensions = map[string]string{
-	"sc":              "http://schema.org#",
-	"commentsEnabled": "sc:Boolean",
-}
+var Extensions = map[string]string{}
 
 // Activity describes actions that have either already occurred, are in the
 // process of occurring, or may occur in the future.
@@ -122,7 +119,7 @@ type Object struct {
 	Tag          []Tag             `json:"tag"`
 
 	// Extensions
-	CommentsEnabled bool `json:"commentsEnabled"`
+	// NOTE: add extensions here
 }
 
 // NewNoteObject creates a basic Note object that includes the public