xen/swiotlb: Remove functions not needed anymore.

Sparse warns us off:
drivers/xen/swiotlb-xen.c:506:1: warning: symbol 'xen_swiotlb_map_sg' was not declared. Should it be static?
drivers/xen/swiotlb-xen.c:534:1: warning: symbol 'xen_swiotlb_unmap_sg' was not declared. Should it be static?

and it looks like we do not need this function at all.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>