Remove deadcode under models/issues (#28536)

Using the Go Official tool `golang.org/x/tools/cmd/deadcode@latest`
mentioned by [go blog](https://go.dev/blog/deadcode).
Just use `deadcode .` in the project root folder and it gives a list of
unused functions. Though it has some false alarms.
This PR removes dead code detected in `models/issues`.

models/issues/assignees_test.go

@@ -18,7 +18,10 @@ func TestUpdateAssignee(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 
 	// Fake issue with assignees
-	issue, err := issues_model.GetIssueWithAttrsByID(db.DefaultContext, 1)
+	issue, err := issues_model.GetIssueByID(db.DefaultContext, 1)
+	assert.NoError(t, err)
+
+	err = issue.LoadAttributes(db.DefaultContext)
 	assert.NoError(t, err)
 
 	// Assign multiple users