Visual Studio projects: Clean up generating gtk.def

Hi,

There were some recent changes to the autotools build system to not distribute gtk.def due to differences in 32-bit and 64-bit Windows builds, that led to this cleanup as the Visual Studio projects already handled this difference duruing the build. From the commit message:

It turns out that we do not need to have separate command lines for running the Visual Studio preprocessor for 32-bit builds and 64-bit builds as _WIN64 is automatically defined by the 64-bit compilers.

This means that we can clean up the project files a bit.

With blessings, thank you!