COMP: Do not compile zlib using WINAPI calling convention

Following commit r21963, windows 32-bit couldn't be be built because
of missing symbol error associated with python, vtk and itk. Ensuring
ZLIB_WINAPI was defined systematically by configuring it inside "zlibDllConfig.h"
was solving python link error but was then causing further link error while
building ITK, VTK (conflicting calling convention fastcall vs stdcall)

Since originally the flag has been added to be able to compile LibArchive
and since now LibArchive supports zlib build with or without ZLIB_WINAPI,
there is NOW no need to force the flag.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21998 3bd1e089-480b-0410-8dfb-8563597acbee