[merge] remove Transport.should_cache

[John Arbash Meinel]

John Arbash Meinel has voted approve.
Status is now: Approved
Comment:
Yeah, I wrote that a while ago (a long, looong time ago).
It was never really used. And we've generally used other ideas for 
caching. (We always cache .kndx, and we only cache inventory.knit when 
we use it for fetching.)

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Ce01316480708142159u33c2f8e2g8c75960fce521d1e%40mail.gmail.com%3E