projects: Remove the Visual C++ 7.1 project

All versions of Microsoft's C/C++ compilers, old and new, can be driven
with scripts/makefile.vcwin*.

As for the newer Visual Studio environments, we have projects/vstudio/.

Importantly, the removal of projects/visualc71/ marks the end of support
for a specialied DLL build (libpng*vb.dll) targeted specifically at
Visual Basic users. Going forward, the "vanilla" libpng18.dll build
should be sufficient for all use cases.