iommufd: Remove iommufd_ref_to_users()

iommufd: Remove iommufd_ref_to_users()

This no longer has any callers, remove the function

Kevin noticed that after commit 99f98a7c0d69 ("iommufd: IOMMUFD_DESTROY
should not increase the refcount") there was only one other user and it
turns out the rework in commit 9227da7816dd ("iommufd: Add
iommufd_access_change_ioas(_id) helpers") got rid of the last one.

Link: https://lore.kernel.org/r/0-v1-abb31bedd888+c1-iommufd_ref_to_users_jgg@nvidia.com
Suggested-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>