CheckIndex should not duplicate SegmentInfos serialization

CheckIndex tries to read the segments file to provide more information about the problem, but it gets almost everything wrong: it reads an int which based on comments and error messages is either the format or the version but this is actually the magic number of the codec header. It looks like this code is about10 years old and hasn't been updated when we added checksums to index files. SegmentInfos does a better job at detecting issues nowadays, so these manual checks can be removed.