storage: remove broken default parameter from isodate()
storage: remove broken default parameter from isodate()

With that syntax, the default timestamp would have been from the time of
the function's definition (not invocation). But all callers are passing
an explicit timestamp anyway.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin