BlockOffsetSharedArray::check_reducing_assertion uses ParGCRareEvent_lock; this code has, after CMS removal, only been in use by Serial GC, so that clause will never be true.

Remove.

inline void BlockOffsetSharedArray::check_reducing_assertion(bool reducing) {
    assert(reducing || !SafepointSynchronize::is_at_safepoint() || init_to_zero() ||
           Thread::current()->is_VM_thread() ||
           Thread::current()->is_ConcurrentGC_thread() ||
           ((!Thread::current()->is_ConcurrentGC_thread()) &&
            ParGCRareEvent_lock->owned_by_self()), "Crack");
}