Revert "Automatically set 'nohide' on referral exports."

This partially reverts commit ec637de16210c1c6fcb3a0df34d7889592f577dc.

Only NFSv4 clients will actually want to see referall points--others are
better off just seeing an empty directory, that they can manually (or
with automount) mount the appropriate filesystem on.

So we want the kernel to automatically traverse only in the v4 case (as
recent kernels do).

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>