SRM: removed dead code

the attempt to truncate blocks was a) not working as it didn't adjust
cklast and b) wrong. Seems the chunk numbers in blocks are way more
trustworthy as the total number of chunks.
Info: provide/show non-zero cadence

cad_avg is including all samples - also those with zero cadence.

added cad_navg which excludes zero cadence values to be comparable with
what others do.