Revert "set vfs.zfs.arc_min to 256M to work around a problem with auto-tuning"

This reverts commit 9cea4792a553772e0742f25915310073b85ed73b.

The original change becomes redundant with hybridlogic/freebsd@4637e2749fa
Merge pull request #4 from hybridlogic/revert-set_arc_min_size-64165932

Revert "set arc min size" change