PrivateStorageio

These pieces are clearly not necessary and removing them makes the system
closure a bit smaller.

There are still many more pieces that *seem* like they should be unnecessary
that get pulled into a build.  However, these are only *build* time
dependencies and it's somewhat harder to get rid of them: it requires either
passing non-default arguments to the Nix expression defining the
derivation (not too hard, only works where supported, determined on a
case-by-case basis), modifying the Nix expressions themselves for each package
to remove the unwanted dependencies (harder, only works where supported,
determined on a case-by-case basis), or actually modifying the upstream
software and/or its build system to eliminate the dependency (very hard,
unlikely to be worth pursuing in general).

Once we consider the runtime closure and the build time closure separately,
our system sizes are not that bad - around 500MB (payment a little smaller,
storage node a little bigger).