cmd/devp2p/internal/ethtest: remove WriteTo method (ethereum#23379)

WriteTo is unused, and go vet warns about it because it
doesn't match the usual signature of the io.WriterTo method.

cmd/devp2p/internal/ethtest/chain.go

@@ -39,16 +39,6 @@ type Chain struct {
 	chainConfig *params.ChainConfig
 }
 
-func (c *Chain) WriteTo(writer io.Writer) error {
-	for _, block := range c.blocks {
-		if err := rlp.Encode(writer, block); err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
 // Len returns the length of the chain.
 func (c *Chain) Len() int {
 	return len(c.blocks)