swiotlb: remove the unused swiotlb_force declaration

The 'swiotlb_force' is removed since commit c6af2aa9 ("swiotlb: make
the swiotlb_init interface more useful").
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/swiotlb.h

@@ -60,7 +60,6 @@ dma_addr_t swiotlb_map(struct device *dev, phys_addr_t phys,
 		size_t size, enum dma_data_direction dir, unsigned long attrs);
 
 #ifdef CONFIG_SWIOTLB
-extern enum swiotlb_force swiotlb_force;
 
 /**
  * struct io_tlb_mem - IO TLB Memory Pool Descriptor