appveyor: don't set "cmake -A" explicitly
appveyor: don't set "cmake -A" explicitly

Because old "Visual Studio ..." generator doesn't support it.

CMAKE_GENERATOR_PLATFORM environment variable sets
CMAKE_GENERATOR_PLATFORM CMake variable implicitly.

komainu8/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,naoa/groonga,groonga/groonga,groonga/groonga,groonga/groonga,naoa/groonga,naoa/groonga,komainu8/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,groonga/groonga