Remove duplicate section in changelog

Failing job: https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/270985350

Authenticating with credentials from job payload (GitLab Registry)
$ scripts/lint-doc.sh
=> Checking for cURL short options...
✖ ERROR: Duplicate versions in CHANGELOG.md:
=> Checking for CHANGELOG.md duplicate entries...
## 11.11.8
Running after script...
$ date
Mon Aug 12 21:00:54 UTC 2019
ERROR: Job failed: exit code 1

It looks like the auto CE-to-EE merge somehow duplicated the 11.11.8 section in CHANGELOG.md.

https://gitlab.com/gitlab-org/gitlab-ee/commit/778d98265cc1b81c0887a0a25cd6493dd6932a32