xen: remove the xen_biovec_phys_mergeable export

BIOVEC_PHYS_MERGEABLE is only called from core block code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/xen/biomerge.c

@@ -20,4 +20,3 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 	return false;
 #endif
 }
-EXPORT_SYMBOL(xen_biovec_phys_mergeable);