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