ARM/io: Remove useless definition of mmiowb()

ARM includes asm-generic/io.h, which provides a dummy definition of
mmiowb() if one isn't already provided by the architecture.

Remove the useless definition.
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm/include/asm/io.h

@@ -281,8 +281,6 @@ extern void _memcpy_fromio(void *, const volatile void __iomem *, size_t);
 extern void _memcpy_toio(volatile void __iomem *, const void *, size_t);
 extern void _memset_io(volatile void __iomem *, int, size_t);
 
-#define mmiowb()
-
 /*
  *  Memory access primitives
  *  ------------------------