arch: Remove dummy mmiowb() definitions from arch code

Now that no driver code is using mmiowb() directly, remove the dummy
definitions remaining in architectures that don't make use of
asm-generic/io.h, as well as the definition in asm-generic/io.h itself.

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

arch/sparc/include/asm/io_64.h

@@ -396,8 +396,6 @@ static inline void memcpy_toio(volatile void __iomem *dst, const void *src,
 	}
 }
 
-#define mmiowb()
-
 #ifdef __KERNEL__
 
 /* On sparc64 we have the whole physical IO address space accessible