logviewer: depreciate sessions info

[pratapvardhan]

I'm not sure how useful sessions (this has nothing to do with session.json, we try to guess # of unique sessions) information is given the nature of apps and user attentions. If we drop this calculation, logviewer.py could be simplified.

@bkamapantula -- how often is sessions used/needed currently? @sanand0 -- a simpler logviewer should make it easier to develop custom variants.

[bkamapantula]

@pratapvardhan sessions aren't used as much. if it's derivable and documented that'll help apps create wrappers.