One more attempt to simplify recommendation-ids

It works as intended this time:

```
user=> (let [recs [1 2 3 4 5] completed (set [2 4 6 8])] (filter #(not (contains? completed %)) recs))
(1 3 5)
user=> (let [recs [1 2 3 4 5] completed (set [2 4 6 8])] (remove completed recs))
(1 3 5)
```