Remove appveyor CI

retire msvc support and corresponding CI. No active development happens on msvc,
and build is much slower or wrong.

gcc (mingw) is our toolchain of choice also on windows, and the latter is tested.

No functional change

appveyor.yml

@@ -1,88 +0,0 @@
-version: 1.0.{build}
-clone_depth: 50
-
-branches:
-  only:
-    - master
-
-# Operating system (build VM template)
-os: Visual Studio 2019
-
-# Build platform, i.e. x86, x64, AnyCPU. This setting is optional.
-platform:
-  - x86
-  - x64
-
-# build Configuration, i.e. Debug, Release, etc.
-configuration:
-  - Debug
-  - Release
-
-matrix:
-  # The build fail immediately once one of the job fails
-  fast_finish: true
-
-# Scripts that are called at very beginning, before repo cloning
-init:
-  - cmake --version
-  - msbuild /version
-
-before_build:
-  - ps: |
-      # Get sources
-      $src = get-childitem -Path *.cpp -Recurse | select -ExpandProperty FullName
-      $src = $src -join ' '
-      $src = $src.Replace("\", "/")
-
-      # Build CMakeLists.txt
-      $t = 'cmake_minimum_required(VERSION 3.17)',
-           'project(Stockfish)',
-           'set(CMAKE_CXX_STANDARD 17)',
-           'set(CMAKE_CXX_STANDARD_REQUIRED ON)',
-           'set (CMAKE_CXX_EXTENSIONS OFF)',
-           'set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR}/src)',
-           'set(source_files', $src, ')',
-           'add_executable(stockfish ${source_files})'
-
-      # Write CMakeLists.txt withouth BOM
-      $MyPath = (Get-Item -Path "." -Verbose).FullName + '\CMakeLists.txt'
-      $Utf8NoBomEncoding = New-Object System.Text.UTF8Encoding $False
-      [System.IO.File]::WriteAllLines($MyPath, $t, $Utf8NoBomEncoding)
-
-      # Obtain bench reference from git log
-      $b = git log HEAD | sls "\b[Bb]ench[ :]+[0-9]{7}" | select -first 1
-      $bench = $b -match '\D+(\d+)' | % { $matches[1] }
-      Write-Host "Reference bench:" $bench
-      $g = "Visual Studio 16 2019"
-      If (${env:PLATFORM} -eq 'x64') { $a = "x64" }
-      If (${env:PLATFORM} -eq 'x86') { $a = "Win32" }
-      cmake -G "${g}" -A ${a} .
-      Write-Host "Generated files for: " $g $a
-
-build_script:
-  - cmake --build . --config %CONFIGURATION% -- /verbosity:minimal
-  - ps: |
-      # Download default NNUE net from fishtest
-      $nnuenet = Get-Content -Path src\evaluate.h | Select-String -CaseSensitive -Pattern "EvalFileDefaultName" | Select-String -CaseSensitive -Pattern "nn-[a-z0-9]{12}.nnue"
-      $dummy = $nnuenet -match "(?<nnuenet>nn-[a-z0-9]{12}.nnue)"
-      $nnuenet = $Matches.nnuenet
-      Write-Host "Default net:" $nnuenet
-      $nnuedownloadurl = "https://tests.stockfishchess.org/api/nn/$nnuenet"
-      $nnuefilepath = "src\${env:CONFIGURATION}\$nnuenet"
-      if (Test-Path -Path $nnuefilepath) {
-            Write-Host "Already available."
-      } else {
-            Write-Host "Downloading $nnuedownloadurl to $nnuefilepath"
-            Invoke-WebRequest -Uri $nnuedownloadurl -OutFile $nnuefilepath
-      }
-
-before_test:
-  - cd src/%CONFIGURATION%
-  - stockfish bench 2> out.txt >NUL
-  - ps: |
-      # Verify bench number
-      $s = (gc "./out.txt" | out-string)
-      $r = ($s -match 'Nodes searched \D+(\d+)' | % { $matches[1] })
-      Write-Host "Engine bench:" $r
-      Write-Host "Reference bench:" $bench
-      If ($r -ne $bench) { exit 1 }