powerpc: Kill unused swiotlb variable

We can kill unused swiotlb variable.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/dma-swiotlb.c

@@ -21,7 +21,6 @@
 #include <asm/dma.h>
 #include <asm/abs_addr.h>
 
-int swiotlb __read_mostly;
 unsigned int ppc_swiotlb_enable;
 
 /*