Removing unneeded query to improve group performance.

As mentioned in #1702, the query in `insertTargetIntoGroup` isn't needed, since both instances where it's used we already know that the target isn't in the group. This means it's safe to remove that query, improving performance dramatically.

Before:

```
BenchmarkPostGroup100-4     	      81	  12629754 ns/op	 2989993 B/op	   52918 allocs/op
BenchmarkPostGroup1000-4    	       6	 189527792 ns/op	29891818 B/op	  528082 allocs/op
BenchmarkPostGroup10000-4   	       1	6203645806 ns/op	299253648 B/op	 5282859 allocs/op
BenchmarkPutGroup100-4      	     100	  10221833 ns/op	 2589165 B/op	   46078 allocs/op
BenchmarkPutGroup1000-4     	       7	 162692432 ns/op	25812440 B/op	  458425 allocs/op
BenchmarkPutGroup10000-4    	       1	7596445084 ns/op	260385808 B/op	 4581569 allocs/op
```

After:

```
BenchmarkPostGroup100-4     	     133	   8848973 ns/op	 2354605 B/op	   42322 allocs/op
BenchmarkPostGroup1000-4    	       9	 112557410 ns/op	23549206 B/op	  422184 allocs/op
BenchmarkPostGroup10000-4   	       1	3414209403 ns/op	235635952 B/op	 4222090 allocs/op
BenchmarkPutGroup100-4      	     147	   8094333 ns/op	 2271297 B/op	   40777 allocs/op
BenchmarkPutGroup1000-4     	       9	 125092124 ns/op	22635067 B/op	  405421 allocs/op
BenchmarkPutGroup10000-4    	       1	5712591900 ns/op	228592920 B/op	 4051316 allocs/op
```

models/group.go

@@ -327,14 +327,11 @@ func insertTargetIntoGroup(tx *gorm.DB, t Target, gid int64) error {
 		}).Error(err)
 		return err
 	}
-	err = tx.Where("group_id=? and target_id=?", gid, t.Id).Find(&GroupTarget{}).Error
-	if err == gorm.ErrRecordNotFound {
 	err = tx.Save(&GroupTarget{GroupId: gid, TargetId: t.Id}).Error
 	if err != nil {
 		log.Error(err)
 		return err
 	}
-	}
 	if err != nil {
 		log.WithFields(logrus.Fields{
 			"email": t.Email,