catfile: Remove `Object[Content|Info]Reader`
We introduced ObjectReader
as a replacement for the existing
Object[Content|Info]Reader
readers. Since we moved to git v2.42, we
no longer use the latter. The former uses 'git-cat-file(1)'s
'--batch-command' mode to read both object info and object contents.
Let's cleanup all the dead code and remove everything around it.
Closes #5655 (closed)