Revert "Temporarily pre-create ~stack/.cache for pip 6"

This reverts commit 0caf2e4dd2be91ef08484d9f1755ffe45480732f.

Turns out the problem was with a "sideways" grenade job in
stable/juno, so no amount of workaround in devstack stable/icehouse
is going to help anyway.

Change-Id: Ieca3df91468b454d3bfae438f95710859fd8f1a0