There is no point in it. We know how many rows a delete_delta file has from ORC and they are all the same type - so no need for AcidStats.
hive.acid.key.index has no value since delete_delta files are never split and are not likely to have more than 1 stripe since they are very small.
Also can remove KeyIndexBuilder.acidStats - we only have 1 type of event per file
if doing this, make sure to fix OrcInputFormat.isOriginal(Reader) and OrcInputFormat.isOriginal(Footer) etc
there is new KeyIndexBuilder("delete") and new KeyIndexBuilder("insert"). The later is needed in
HIVE-16812, the former can be removed.