Use the --purge flag in doc/install/kubernetes/gitlab_omnibus.md
Use the --purge flag in doc/install/kubernetes/gitlab_omnibus.md

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6101.

Signed-off-by: Rémy Coutable <4ea0184b9df19e0786dd00b28e6daa4d26baeb3e@rymai.me>

jirutka/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,dreampet/gitlab,iiet/iiet-git,dreampet/gitlab,axilleas/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,stoplightio/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq