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/ping.go

@@ -65,11 +65,3 @@ func (p *Ping) Command() uint32 {
 func (p Ping) MaxPayloadLength(uint32) uint32 {
 	return 65532
 }
-
-// Validate performs any necessary sanity checks to ensure all fields present
-// on the Ping are valid.
-//
-// This is part of the lnwire.Message interface.
-func (p *Ping) Validate() error {
-	return nil
-}