Remove C++/CX from the codebase

C++/WinRT is now the recommended way to write UWP apps. Also C++/CX is not compatible with C++20.

I'm trying to migrate our project to C++20 which have dependency on cpprestsdk, but unfortunately the linker fails saying the standard libraries are incompatible. I'm getting this error:

vccorlib_lib_should_be_specified_before_msvcrt_lib_to_linker': value '1' doesn't match value '0' in msvcrt.lib