Remove release docs from the CE repository
More detailed, updated, and accurate versions of these docs now live in https://gitlab.com/gitlab-org/release-tools
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2346
[ci skip]
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
More detailed, updated, and accurate versions of these docs now live in https://gitlab.com/gitlab-org/release-tools
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2346
[ci skip]