Move clearlabels from models to issue service (#8326)

* move clearlabels from models to issue service

* improve code

* Apply suggestions from code review

Co-Authored-By: zeripath <art27@cantab.net>

services/issue/label.go

@@ -0,0 +1,21 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package issue
+
+import (
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/notification"
+)
+
+// ClearLabels clears all of an issue's labels
+func ClearLabels(issue *models.Issue, doer *models.User) (err error) {
+	if err = issue.ClearLabels(doer); err != nil {
+		return
+	}
+
+	notification.NotifyIssueClearLabels(doer, issue)
+
+	return nil
+}