Removes dev client from security tasks
What does this MR do?
Omnibus is currently moving its security development to GitLab Security, so it won't be longer necessary to validate and merge MR on dev.gitlab.org.
Also modifies the MergeRequestsValidator#projects_to_verify
to return only GitLab Security and
Omnibus GitLab Security
Related to gitlab-com/gl-infra/delivery#692 (closed)