JCLOUDS-622: Remove last vestiges of InputSupplier

Guava 16 deprecated InputSupplier and a future release will remove it.
> By that, do you mean merging it to master once the 1.8.0 branch has been created, as described in your first comment? Or something else?
> 
> The change itself looks fine to me, needless to say.

Yes, merge after 1.8.x branches.