Remove contrib/zfs directory

A WIP pull request or issue for discussion would be a better avenue for collaboration and discussion of a ZFS backend.

@gurjeet any objections?  It doesn't appear that you're actively working on your driver, and the "contrib/zfs" directory gives people the impression that a ZFS backend is being actively developed.  If this is not correct and your driver is actively developed, why not submit a PR and finally work on getting it mainlined?