[C++] Drop support for C++/CLI

[kou]

Describe the enhancement requested

We're caring about C++/CLI by not using nullptr/<mutex> in public header. (But we don't have CI for C++/CLI.)

But it seems that there is no Arrow + C++/CLI user ( #45674 (comment) ) now. We have the C# implementation now. So we can drop support for C++/CLI.

Component(s)

C++

[pitrou]

@raulcd @kou Should we just do this?

[kou]

Yes.

We need to remove

arrow/cpp/src/arrow/util/macros.h

Lines 119 to 130 in 7129321

	// ----------------------------------------------------------------------
	// C++/CLI support macros (see ARROW-1134)
	#ifndef NULLPTR
	# ifdef __cplusplus_cli
	# define NULLPTR __nullptr
	# else
	# define NULLPTR nullptr
	# endif
	#endif // ifndef NULLPTR

, https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/mutex.h and https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/mutex.cc .

[raulcd]

There's a repeated link, I think you wanted to point to the mutex header file, maybe the comment specifically:

arrow/cpp/src/arrow/util/mutex.h

Lines 28 to 31 in 9a21925

	/// A wrapper around std::mutex since we can't use it directly in
	/// public headers due to C++/CLI.
	/// https://docs.microsoft.com/en-us/cpp/standard-library/mutex#remarks
	class ARROW_EXPORT Mutex {

[kou]

Oh, sorry... I've fixed the .h link...

We can remove mutex.h entirely.