Avoid unary minus over unsigned type in ObjectSynchronizer::dec_in_use_list_ceiling

SonarCloud complains:
  Unary minus should not be applied to an unsigned expression

Here:

void ObjectSynchronizer::dec_in_use_list_ceiling() {
  Atomic::add(&_in_use_list_ceiling, -AvgMonitorsPerThreadEstimate);
}

We can instead use Atomic::sub.