It seems that the `machinectl` backing store is prone to errors and it
is causing instability in the gate since it was introduced. Also, the
default value for it is "dir" which means 99% of deployments will be
using that value.
We should be gating for the most commonly used case, therefore, by
removing it, we're gating with a much similar environment as the one
used by our users.
Change-Id: Icc099981f2a216d6c4d10f8a23f56cac32cc932f
8a18a23511