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`.
user1,err:=user_model.GetUserByID(db.DefaultContext,1)// This user is already assigned (see the definition in fixtures), so running UpdateAssignee should unassign him