x86/io: Remove useless definition of mmiowb()

x86 maps mmiowb() to barrier(), but this is superfluous because a
compiler barrier is already implied by spin_unlock(). Since x86 also
includes asm-generic/io.h in its asm/io.h file, remove the definition
entirely and pick up the dummy definition from core code.
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/x86/include/asm/io.h

@@ -90,8 +90,6 @@ build_mmio_write(__writel, "l", unsigned int, "r", )
 #define __raw_writew __writew
 #define __raw_writel __writel
 
-#define mmiowb() barrier()
-
 #ifdef CONFIG_X86_64
 
 build_mmio_read(readq, "q", u64, "=r", :"memory")