[Cake] Dropping last_len from stats collection & display.

[Jonathan Morton]

> On 1 Jun, 2016, at 14:40, Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:
> 
> Not updating another variable (memory write) for each packet may help cake's CPU usage a little.

I’ll think about this one.

I don’t think it should gate the LEDE merge (nor should it require a stats version bump), since as long as the entry remains in the stats struct, it will simply remain zero if not written.

In future the space could be reused, in which case a version bump would be appropriate.

 - Jonathan Morton