build: Removed WIN64 definition from the libcurl Visual Studio projects

Removed the WIN64 pre-processor definition from the libcurl project
files as:

* WIN64 is not used in our source code
* The curl projects files don't define it
* It isn't required by or used in the platform SDK
* For backwards compatability curl_setup.h defines WIN32
* The compiler automatically defines _WIN64 for x64 builds

Historically Visual Studio projects have defined WIN32, in addition to
the compiler defined _WIN32 definition, and I had incorrectly changed
that to WIN64 for the x64 libcurl builds but not in the curl projects.

As such, it is questionable whether this should be defined or not. For
more information see the following cache of a discussion that took
place on the microsoft.public.vc.mfc newsgroup:

http://www.tech-archive.net/Archive/VC/microsoft.public.vc.mfc/2008-06/msg00074.html