MINOR: [C++] Fix compiler warnings in Clang

[jaepil]

This PR fixes compiler warnings in Clang.

❯ ninja clean && ninja
[1/1] Cleaning all built files...
Cleaning... 0 files.
[289/391] Building CXX object src/arrow/acero/CMakeFiles/arrow_acero_objlib.dir/asof_join_node.cc.o
/Users/jaepil/work/appspand/deps/arrow/cpp/src/arrow/acero/asof_join_node.cc:782:17: warning: private field 'node_' is not used [-Wunused-private-field]
  782 |   AsofJoinNode* node_;
      |                 ^
/Users/jaepil/work/appspand/deps/arrow/cpp/src/arrow/acero/asof_join_node.cc:784:10: warning: private field 'index_' is not used [-Wunused-private-field]
  784 |   size_t index_;
      |          ^
2 warnings generated.
[391/391] Linking CXX static library release/libarrow_substrait.a

[kou]

Could you open a new issue for this?
This is not a MINOR change. See also our MINOR definition: https://github.com/apache/arrow/blob/main/CONTRIBUTING.md#Minor-Fixes

Could you use our PR template instead of removing it entirely?

Could you show how to reproduce this?
What is your Clang version? How did you run cmake?

[jaepil]

Could you open a new issue for this? This is not a MINOR change. See also our MINOR definition: https://github.com/apache/arrow/blob/main/CONTRIBUTING.md#Minor-Fixes

Could you use our PR template instead of removing it entirely?

Could you show how to reproduce this? What is your Clang version? How did you run cmake?

No problem. I should have read the CONTRIBUTING.md more carefully. I will update the PR description and create an issue later today.

Thanks for the feedback!

[zanmato1984]

Shall we do the same thing to these two members as what we did to MemoStore, as the changes I proposed?

Of course, this requires some other cleanup too.

Thank you.

[zanmato1984]

Suggested change

	// Owning node
	#ifndef NDEBUG
	// Owning node

[zanmato1984]

Suggested change

	[[maybe_unused]] AsofJoinNode* node_;
	AsofJoinNode* node_;

[zanmato1984]

Suggested change

	[[maybe_unused]] size_t index_;
	size_t index_;
	#endif

[mapleFU]

@zanmato1984 so these two members are only used in debug mode?

[zanmato1984]

@zanmato1984 so these two members are only used in debug mode?

Yes, I think so.