vpx_integer.h: fix incorrect type emulation

The previous definition of uintptr_t was incorrect on x64 with MSVC.
Also, the ARMV6 version of int_fast16_t was defined as unsigned for
some reason. Since the fast types are currently unused, just remove
them.

Change-Id: Idd73f77a989c77feedcb4a6802ae6bd37324ed40

vpx/vpx_integer.h

@@ -29,16 +29,8 @@ typedef signed __int64   int64_t;
 typedef unsigned __int64 uint64_t;
 #endif
 
-#ifdef HAVE_ARMV6
-typedef unsigned int int_fast16_t;
-#else
-typedef signed short int_fast16_t;
-#endif
-typedef signed char int_fast8_t;
-typedef unsigned char uint_fast8_t;
-
 #ifndef _UINTPTR_T_DEFINED
-typedef unsigned int   uintptr_t;
+typedef size_t uintptr_t;
 #endif
 
 #else