Remove deprecated "Project Git Hooks" from API in 9.0

Remove Project Git Hooks from the API in 9.0, they've been renamed push_rules: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/api/project_git_hook.rb#L1

added ~481018 label

closed

Done in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1301