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