We should delete global_scrap_object code

[bneradt]

While doing some logging work, I noticed that our code instantiates a so-called scrap LogObject:

https://github.com/apache/trafficserver/blob/b85d9fd8481bc55547aade92f5b9cf7a8db5099f/proxy/logging/Log.cc#L1091=#L1095

This object is never written to, thus it is dead code. Digging through our history, it looks like this used to be associated with log collation, a feature which was removed in #5353. Since that no longer exists, we can simply remove the scrap log object from our code.