CI: Fix AppVeyor x64 (64bit) build

Architecture not part of `-G` anymore, maybe `-A x64`

Author: ax3l

.appveyor.yml

@@ -101,7 +101,7 @@ before_build:
   # Compiler & Generator Selection
   - cmd: if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2017" set OPENPMD_CMAKE_GENERATOR=Visual Studio 15 2017
   - cmd: if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2019" set OPENPMD_CMAKE_GENERATOR=Visual Studio 16 2019
-  - cmd: if "%TARGET_ARCH%"=="x64" set OPENPMD_CMAKE_GENERATOR=%OPENPMD_CMAKE_GENERATOR% Win64
+#  - cmd: if "%TARGET_ARCH%"=="x64" set OPENPMD_CMAKE_GENERATOR=%OPENPMD_CMAKE_GENERATOR% Win64
 #  - cmd: if "%TARGET_ARCH%"=="x86" "C:\Program Files (x86)\Microsoft Visual Studio 15.9\VC\vcvarsall.bat" x86
 #  - cmd: if "%TARGET_ARCH%"=="x64" "C:\Program Files (x86)\Microsoft Visual Studio 15.9\VC\vcvarsall.bat" amd64