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 go-gitea#5318

Author: nougad

models/migrations/v67.go

@@ -5,6 +5,8 @@
 package migrations
 
 import (
+	"fmt"
+
 	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/go-xorm/xorm"
@@ -70,6 +72,13 @@ func removeStaleWatches(x *xorm.Engine) error {
 		return err
 	}
 
+	var issueWatch IssueWatch
+	if exist, err := sess.IsTableExist(&issueWatch); err != nil {
+		return fmt.Errorf("IsExist IssueWatch: %v", err)
+	} else if !exist {
+		return nil
+	}
+
 	repoCache := make(map[int64]*Repository)
 	err := x.BufferSize(setting.IterateBufferSize).Iterate(new(Watch),
 		func(idx int, bean interface{}) error {