cache: get rid of mdb_env_sync()
Explicit cache sync does not seem necessary and might be counterproductive, see other comments in the thread:
The following discussion from !1042 should be addressed:
- [ ] @pspacek started a [discussion](https://gitlab.nic.cz/knot/knot-resolver/-/merge_requests/1042#note_169608): (+1 comment)
> Out of curiosity, why the sync is necessary here?
issue