cpu_manager: Remove unused preemption_count variable

Shrinks the data structure by 8 bytes.

src/core/cpu_manager.h

@@ -100,7 +100,6 @@ private:
     bool is_async_gpu{};
     bool is_multicore{};
     std::atomic<std::size_t> current_core{};
-    std::size_t preemption_count{};
     std::size_t idle_count{};
     static constexpr std::size_t max_cycle_runs = 5;