contrib/buildsystems: drop support for building .vcproj/.vcxproj files

Before we had CMake support, the only way to build Git in Visual Studio
was via this hacky `generate` script.

For a while I tried to fix whenever things got broken, in particular to
allow building confidence in embargoed releases by running the CI builds
in Azure Pipelines in a private Azure DevOps project. I even carried the
patches in Git for Windows with the intention of upstreaming them,
eventually.

However, it is a lot of work with too little benefit. CMake is much
better supported by Visual Studio. So let's drop this hacky script (plus
support code).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho