refactor: don't return an error from Node._hash

There's no need because the hashing can never fail.

import.go

@@ -56,9 +56,7 @@ func newImporter(tree *MutableTree, version int64) (*Importer, error) {
 
 // writeNode writes the node content to the storage.
 func (i *Importer) writeNode(node *Node) error {
-	if _, err := node._hash(node.nodeKey.version); err != nil {
-		return err
-	}
+	node._hash(node.nodeKey.version)
 	if err := node.validate(); err != nil {
 		return err
 	}

mutable_tree.go

@@ -1025,10 +1025,7 @@ func (tree *MutableTree) saveNewNodes(version int64) error {
 			}
 		}
 
-		_, err = node._hash(version)
-		if err != nil {
-			return nil, err
-		}
+		node._hash(version)
 		return node.nodeKey.GetKey(), nil
 	}
 

node.go

@@ -126,10 +126,7 @@ func MakeNode(nk, buf []byte) (*Node, error) {
 		}
 		node.value = val
 		// ensure take the hash for the leaf node
-		if _, err := node._hash(node.nodeKey.version); err != nil {
-			return nil, fmt.Errorf("calculating hash error: %v", err)
-		}
-
+		node._hash(node.nodeKey.version)
 	} else { // Read children.
 		node.hash, n, err = encoding.DecodeBytes(buf)
 		if err != nil {
@@ -417,18 +414,18 @@ func (node *Node) getByIndex(t *ImmutableTree, index int64) (key []byte, value [
 
 // Computes the hash of the node without computing its descendants. Must be
 // called on nodes which have descendant node hashes already computed.
-func (node *Node) _hash(version int64) ([]byte, error) {
+func (node *Node) _hash(version int64) []byte {
 	if node.hash != nil {
-		return node.hash, nil
+		return node.hash
 	}
 
 	h := sha256.New()
 	if err := node.writeHashBytes(h, version); err != nil {
-		return nil, err
+		return nil
 	}
 	node.hash = h.Sum(nil)
 
-	return node.hash, nil
+	return node.hash
 }
 
 // Hash the node and its descendants recursively. This usually mutates all

util.go

@@ -30,10 +30,7 @@ func printNode(ndb *nodeDB, node *Node, indent int) error {
 		printNode(ndb, rightNode, indent+1) //nolint:errcheck
 	}
 
-	hash, err := node._hash(node.nodeKey.version)
-	if err != nil {
-		return err
-	}
+	hash := node._hash(node.nodeKey.version)
 
 	fmt.Printf("%sh:%X\n", indentPrefix, hash)
 	if node.isLeaf() {