m68k/io: Remove useless definition of mmiowb()

m68k 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: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/m68k/include/asm/io_mm.h

@@ -377,8 +377,6 @@ static inline void isa_delay(void)
 #define writesw(port, buf, nr)    raw_outsw((port), (u16 *)(buf), (nr))
 #define writesl(port, buf, nr)    raw_outsl((port), (u32 *)(buf), (nr))
 
-#define mmiowb()
-
 #ifndef CONFIG_SUN3
 #define IO_SPACE_LIMIT 0xffff
 #else