[win32] Remove MemoryBarrier() fallback implementation

I added these because the older mingw32 toolchain didn't have
MemoryBarrier().  The newer mingw-w64 toolchain however has.
As reported by John Emmas this was causing build failure with
MSVC (on glib) because of inline issues.  But that reminded me
that we may be taking this path even if the system implements
MemoryBarrier as a function, which is a waste.  So, just remove
it.

src/hb-atomic-private.hh

@@ -47,22 +47,10 @@
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 
-/* mingw32 does not have MemoryBarrier.
- * MemoryBarrier may be defined as a macro or a function.
- * Just make a failsafe version for ourselves. */
-#ifdef MemoryBarrier
-#define HBMemoryBarrier MemoryBarrier
-#else
-static inline void HBMemoryBarrier (void) {
-  long dummy = 0;
-  InterlockedExchange (&dummy, 1);
-}
-#endif
-
 typedef LONG hb_atomic_int_t;
 #define hb_atomic_int_add(AI, V)	InterlockedExchangeAdd (&(AI), (V))
 
-#define hb_atomic_ptr_get(P)		(HBMemoryBarrier (), (void *) *(P))
+#define hb_atomic_ptr_get(P)		(MemoryBarrier (), (void *) *(P))
 #define hb_atomic_ptr_cmpexch(P,O,N)	(InterlockedCompareExchangePointer ((void **) (P), (void *) (N), (void *) (O)) == (void *) (O))