MINOR: [C++] Remove obsolete todo about NaN handling at compare.cc #4039

Job	Run time
check-labels / Check labels	28s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	0s
ODBC	0s
ARM64 Windows 11 MSVC	0s
matrix.title	0s
JNI macOS	0s
report-extra-cpp	0s
	28s

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