Remove in-memory state implementation

We originally added this in the *very early* days of Podman,
before a proper persistent state was written, so we had something
to test with. It was retained after the original SQLite state
(and current BoltDB state) were written so it could be used for
testing Libpod in unit tests with no requirement for on-disk
storage. Well, such unit tests never materialized, and if we were
to write some now the requirement to have a temporary directory
for storing data on disk is not that bad. I can basically
guarantee there are no users of this in the wild because, even if
you managed to figure out how to configure it when we don't
document it, it's completely unusable with Podman since all your
containers and pods will disappear every time Podman exits.

Given all this, and since it's an ongoing maintenance burden I no
longer wish to deal with, let's just remove it.

Signed-off-by: Matthew Heon <[email protected]>