Fixes #5848

* 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 #5318

* Skip remove stale watcher migration if not required

Otherwise the migration will fail if executed from a older database
version without multiple IssueWatch feature.

```
2018/11/11 23:51:14 [I] [SQL] SELECT DISTINCT "issue_watch"."user_id", "issue"."repo_id" FROM "issue_watch" INNER JOIN issue ON issue_watch.issue_id = issue.id WHERE (issue_watch.is_watching = $1) LIMIT 50 []int
[...itea/routers/init.go:60 GlobalInit()] [E] Failed to initialize ORM engine: migrate: do migrate: pq: relation "issue_watch" does not exist
```

see #5318