Rename, remove or migrate redundant and deprecated data from database and from codebase

• https://gitlab.com/gitlab-org/gitlab-ce/issues/20995
• https://gitlab.com/gitlab-org/gitlab-ce/issues/21097
• https://gitlab.com/gitlab-org/gitlab-ce/issues/22003
• https://gitlab.com/gitlab-org/gitlab-ce/issues/23993
• https://gitlab.com/gitlab-org/gitlab-ce/issues/25684
• https://gitlab.com/gitlab-org/gitlab-ce/issues/27777