Remove stats#show action.  Hasn't been used since 2012.

I started to look into cleaning up the stats#show action's javascript, only to find that the action has been commented out since 2012.

https://github.com/rubygems/rubygems.org/commit/56029316b98fd319b30063d5111f173b20a9c2d9
https://github.com/rubygems/rubygems.org/commit/26bc32fbba37087cf5a6c6a44a2c5da9d20c2c32

Given that we're not really using it, I'm proposing we remove it.