hscontrol: use Updates() instead of Save() for partial updates

Changed UpdateUser and re-registration flows to use Updates() which only
writes modified fields, preventing unintended overwrites of unchanged fields.

Also updated UsePreAuthKey to use Model().Update() for single field updates
and removed unused NodeSave wrapper.

hscontrol/db/node.go

@@ -452,13 +452,6 @@ func NodeSetMachineKey(
 	}).Error
 }
 
-// NodeSave saves a node object to the database, prefer to use a specific save method rather
-// than this. It is intended to be used when we are changing or.
-// TODO(kradalby): Remove this func, just use Save.
-func NodeSave(tx *gorm.DB, node *types.Node) error {
-	return tx.Save(node).Error
-}
-
 func generateGivenName(suppliedName string, randomSuffix bool) (string, error) {
 	// Strip invalid DNS characters for givenName
 	suppliedName = strings.ToLower(suppliedName)