GH-48245: [C++][Parquet] Simplify GetVlqInt #3244

Job	Run time
check-labels / Check labels	30s
ARM64 Windows 11 MSVC	-1s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	-1s
matrix.title	-1s
JNI macOS	-1s
report-extra-cpp	0s
	26s

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