Remove the Appveyor Visual Studio builds that duplicate GitHub Action ones

[claremacrae]

"Visual Studio 2017" and "Visual Studio 2019" builds are being done in both Appveyor and GitHub Actions.

The GitHub Actions ones are finishing much more quickly than the Appveyor ones, which are running in sequence. This means that if there are several pushes in quick succession, it can take a long time to get the final Appveyor results, which now include minnow/MSYS and Cygwin builds.

I've just checked that the identical VisualStudio builds are being used:

"Visual Studio 2017"

• https://ci.appveyor.com/project/isidore/approvaltests-cpp/builds/30060475/job/ehgo8a9fs0hlmf6r#L170
  • Microsoft (R) Build Engine version 15.9.21+g9802d43bc3 for .NET Framework
  • Microsoft (R) C/C++ Optimizing Compiler Version 19.16.27034 for x86
• https://github.com/approvals/ApprovalTests.cpp/runs/385929816#step:4:9
  • Microsoft (R) Build Engine version 15.9.21+g9802d43bc3 for .NET Framework
  • Microsoft (R) C/C++ Optimizing Compiler Version 19.16.27034 for x86

"Visual Studio 2019"

• https://ci.appveyor.com/project/isidore/approvaltests-cpp/builds/30060475/job/9lsj51rn3hdmsrpt#L218
  • Microsoft (R) Build Engine version 16.4.0+e901037fe for .NET Framework
  • Microsoft (R) C/C++ Optimizing Compiler Version 19.24.28314 for x64
• https://github.com/approvals/ApprovalTests.cpp/runs/385929805#step:4:4
  • Microsoft (R) Build Engine version 16.4.0+e901037fe for .NET Framework
  • Microsoft (R) C/C++ Optimizing Compiler Version 19.24.28314 for x64

So I propose to remove the Appveyor Visual Studio builds.

[claremacrae]

This was fixed in 58fcbf11453fab5fa652f7550d2c2540f004e2e0 which is now merged to the main branch.