vnstat: Update to 1.11

Update vnstat to version 1.11, which also contains
the vnstati binary, for making graphs.

Remove the separate vnstati package.

This commit does not contain anything for doing
backups before upgrading, since I do not know
how that works.

The source for vnstat-1.11 has been downloaded from :
http://humdi.net/vnstat/vnstat-1.11.tar.gz
The changelog for vnstat-11 is here :
http://humdi.net/vnstat/CHANGES