[#1247] session: Remove unused `storage.ErrNotFound` error

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/session/storage/storage.go

@@ -1,7 +1,6 @@
 package storage
 
 import (
-	"errors"
 	"sync"
 
 	"github.com/mr-tron/base58"
@@ -19,8 +18,6 @@ type TokenStore struct {
 	tokens map[key]*PrivateToken
 }
 
-var ErrNotFound = errors.New("private token not found")
-
 // New creates, initializes and returns a new TokenStore instance.
 //
 // The elements of the instance are stored in the map.