swiotlb-xen: drop DEFAULT_NSLABS

It was introduced by 4035b43d ("xen-swiotlb: remove xen_set_nslabs")
and then not removed by 2d29960a ("swiotlb: dynamically allocate
io_tlb_default_mem").

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Link: https://lore.kernel.org/r/15259326-209a-1d11-338c-5018dc38abe8@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>