Stats: Trash pickup of old localStore usage.

[timmyc]

This PR seeks to remove a few remaining uses of store within Stats code. At one point we wanted to keep track of the dates users had been visiting stats pages - statsVisitDates is no longer used and has been removed from the controller logic here.

The other bit of logic is to track which tab in the stats chart was last focused. As we do further redux work in stats, we might consider adding a ui state tree for this aspect of stats, but I feel defaulting to 'Views' in the stats chart is fine personally.

To Test

• Load up the stats overview page http://calypso.localhost:3000/stats - ensure it loads as expected
• Click through to a site listed there, ensure the site stats page loads properly

[matticbot]

Test live: https://calypso.live/?branch=update/stats/remove-stats-visits

[timmyc]

/cc @mtias this pulls out the last usage of localStorage in stats.

[mtias]

Looks good to me. (Switching sites seems a bit laggy to me on this PR, but may just be my browser that's been crawling lately.)