Remove Gitlab::SidekiqDaemon::MemoryKiller

We enabled the Memory Watchdog as a default and replaced the old Daemon Sidekiq Memory Killer. The watchdog is backward compatible with the old memory killer, so there were no breaking changes. We want to keep the old Sidekiq Memory Killer until %16.0, just in case some of our self-managed customers face some issues. We should remove Daemon Sidekiq Memory Killer in %16.0