[C++] Remove deprecated ObjectType and FileStatistics

[AlenkaF]

Describe the enhancement requested

Remove deprecated ObjectType and FileStatistics in io/hdfs.h in favour of FileSystem API in arrow::fs.

Component(s)

C++

[amoeba]

Hi @AlenkaF, is it okay if we move this into the 22.0.0 milestone?

[AlenkaF]

Oh yes! I thought I already changed the milestone - will do it now.