MS_WIN64 no longer needed on Appveyor MinGW for Python

Not sure why - 64bit must be default now.
Fixes Appveyor and Python C++20 tests as the -std=c++20 flags were
not being passed to the compiler.

appveyor.yml

@@ -138,9 +138,6 @@ install:
       $env:MSYSTEM="MINGW$env:MBITS" # This is important for msys2
       $env:CC="gcc"
       $env:CXX="g++"
-      if ($env:MBITS -eq "64" -and $env:SWIGLANG -eq "python") {
-        $env:CHECK_OPTIONS2="CFLAGS+=-DMS_WIN64 CXXFLAGS+=-DMS_WIN64"
-      }
     }
 - if "%OSVARIANT%"=="" bash -c "cd /usr/bin && curl --retry 15 -s -L https://github.com/swig/cccl/archive/cccl-1.2.tar.gz | tar -xz --strip 1 cccl-cccl-1.2/cccl"
 - if "%OSVARIANT%"=="" call "%VCVARSBAT%" %VCVARSARG%