lnwire: remove the Validate method from the Message interface

This commit revues the Validate method from the Message interface as
the method is no longer used and is a relic from an older version of
the codebase.

lnwire/node_announcement.go

@@ -109,15 +109,6 @@ type NodeAnnouncement struct {
 // lnwire.Message interface.
 var _ Message = (*NodeAnnouncement)(nil)
 
-// Validate performs any necessary sanity checks to ensure all fields present
-// on the NodeAnnouncement are valid.
-//
-// This is part of the lnwire.Message interface.
-func (a *NodeAnnouncement) Validate() error {
-	// TODO(roasbeef): move validation to discovery service
-	return nil
-}
-
 // Decode deserializes a serialized NodeAnnouncement stored in the passed
 // io.Reader observing the specified protocol version.
 //