Remove documentation on compiling OpenSSH for CentOS 6
What does this MR do and why?
CentOS 6 is no longer supported as of GitLab 13.6:
- https://about.gitlab.com/releases/2020/11/22/gitlab-13-6-released/#end-of-support-for-centos-6
- omnibus-gitlab!4866 (merged)
We can drop these instructions also because most users aren't compiling a custom version of OpenSSH in any case.
Edited by Stan Hu