[RFC] remove versioned_file.enable_cache

[Ian Clatworthy]

John Arbash Meinel wrote:
> Robert Collins wrote:
>> the enable_cache method seems to be only called once, for knit repositor
>> fetching. I think we can safely remove the cache logic entirely as packs
>> are the default. (That or make the cache be a decorator/optional layer).

> So if you want to refactor it so that it is only enabled for Inventories
> when fetching with Knit format repositories, I'm okay with that. But I
> don't think we want such a significant regression on everyone who hasn't
> upgraded yet.

I use vf.enable_cache() inside the fast-import plugin but just for the
inventory knit. I can't remember the performance gain exactly but it was
noticeable.