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/message.go

@@ -84,7 +84,6 @@ type Message interface {
 	Encode(io.Writer, uint32) error
 	Command() uint32
 	MaxPayloadLength(uint32) uint32
-	Validate() error
 }
 
 // makeEmptyMessage creates a new empty message of the proper concrete type