Remove field from migration to support upgrades from older version

That will ensure the field does not get queried in the Select if it does
not exist yet:

```
[I] [SQL] SELECT "id", "repo_id", "index", "poster_id", "name", "content", "milestone_id", "priority", "assignee_id", "is_closed", "is_pull", "num_comments", "ref", "deadline_unix", "created_unix", "updated_unix
[...itea/routers/init.go:60 GlobalInit()] [E] Failed to initialize ORM engine: migrate: do migrate: pq: column "ref" does not exist
```

see go-gitea#5318

Author: nougad

models/migrations/v64.go

@@ -26,7 +26,6 @@ func addMultipleAssignees(x *xorm.Engine) error {
 		IsClosed    bool  `xorm:"INDEX"`
 		IsPull      bool  `xorm:"INDEX"` // Indicates whether is a pull request or not.
 		NumComments int
-		Ref         string
 
 		DeadlineUnix util.TimeStamp `xorm:"INDEX"`
 		CreatedUnix  util.TimeStamp `xorm:"INDEX created"`