We don't test not using the cache and I'm not aware of anyone using this, it
would be hard to with modern bitbake.
Drop the conditional code and simply error if CACHE isn't set.
(Bitbake rev: 063ffe699bc5fc23174643dfedb66864cacfcff8)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>