Remove roolout issue urs since it's ops flag

config/feature_flags/ops/sidekiq_memory_killer_read_only_mode.yml

 ---
 name: sidekiq_memory_killer_read_only_mode
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/98519
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/374756
 milestone: '15.5'
 type: ops
 group: group::application performance