[sleep_strategy/Stats] remove old obsolete stats member
Before 713c0f04 emper::sleep_strategy::Stats
had the actual stats member but with 713c0f04
WorkerStats
are introduced which have the actual stats member but I
apparently forgot to remove the old now obsolete members.
Thanks clang for detecting this!
See: https://gitlab.cs.fau.de/aj46ezos/emper/-/jobs/456981