Discussion counter is wrong on issue page
### Steps to reproduce
- Open any issue with system notes.
### Example Project
https://gitlab.com/gitlab-org/gitlab-ce/issues/58790
### What is the current *bug* behavior?
The counter shown is much higher than the number of comments. It probably includes the number of system notes:

### What is the expected *correct* behavior?
Only the number of comments is shown.
issue