remove dependency on unittest2 module

remove dependency on unittest2 module

The dependency of fio-histo-log-pctiles.py on unittest2 module was causing heartburn
for pbench community so I removed the dependency.
If unittest2 python package is installed and unittest2 module can be imported,
that's fine, we'll use it if requested,
but if it's not, that's fine too, you can still use the program.