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/powerpc/include/asm/mmiowb.h

@@ -11,8 +11,6 @@
 #define arch_mmiowb_state()	(&local_paca->mmiowb_state)
 #define mmiowb()		mb()
 
-#else
-#define mmiowb()		do { } while (0)
 #endif /* CONFIG_MMIOWB */
 
 #include <asm-generic/mmiowb.h>