GH-50752: [C++][Compute] Fix unused variable warning when ARROW_WITH_RE2 is disabled #8602

Job	Run time
check-labels / Check labels	5s
check-enabled	2s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	0s
matrix.title	0s
JNI macOS	0s
ARM64 Windows 11 MSVC	0s
JNI Windows	0s
ODBC Windows	0s
ODBC Linux	0s
ODBC ${{ matrix.build-type }} ${{ matrix.architecture }} macOS ${{ matrix.macos-version }}	0s
ODBC nightly	0s
report-extra-cpp	0s
	7s

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