MINOR: [C++] Remove obsolete todo about NaN handling at compare.cc

[HyukjinKwon]

Rationale for this change

52d615d added this todo.

arrow/cpp/src/arrow/compare.cc

Line 64 in 52d615d

// TODO also handle HALF_FLOAT NaNs

After 4 years later, 72d20ad implemented the logic

arrow/cpp/src/arrow/compare.cc

Line 92 in 37c40fb

if (Flags::nans_equal && std::isnan(x) && std::isnan(y)) {

arrow/cpp/src/arrow/compare.cc

Line 116 in 37c40fb

if (Flags::nans_equal && f_x.is_nan() && f_y.is_nan()) {

This is being tested in

arrow/cpp/src/arrow/array/array_test.cc

Lines 2314 to 2316 in 37c40fb

	CheckFloatingNanEquality<FloatType>();
	CheckFloatingNanEquality<DoubleType>();
	CheckFloatingNanEquality<HalfFloatType>();

What changes are included in this PR?

This PR removes the obsolete todo about NaN handling at compare.cc.

Are these changes tested?

No, I did not test.

Are there any user-facing changes?

No.