service: remove no longer used Sign/Verify methods and functions

service/sign.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"crypto/ecdsa"
+	"sync"
 
 	crypto "github.com/nspcc-dev/neofs-crypto"
 )
@@ -11,6 +12,12 @@ type keySign struct {
 	sign []byte
 }
 
+var bytesPool = sync.Pool{
+	New: func() interface{} {
+		return make([]byte, 5<<20)
+	},
+}
+
 // GetSignature is a sign field getter.
 func (s keySign) GetSignature() []byte {
 	return s.sign