arm64/io: Remove useless definition of mmiowb()

arm64 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/arm64/include/asm/io.h

@@ -124,8 +124,6 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
 #define __io_par(v)		__iormb(v)
 #define __iowmb()		wmb()
 
-#define mmiowb()		do { } while (0)
-
 /*
  * Relaxed I/O memory access primitives. These follow the Device memory
  * ordering rules but do not guarantee any ordering relative to Normal memory