GH-48245: [C++][Parquet] Simplify GetVlqInt #54494

Job	Run time
AMD64 macOS 15-intel Python 3	29m 54s
AMD64 Conda Python 3.11 Without Pandas	7m 13s
AMD64 Conda Python 3.10 Pandas 1.3.4	8m 3s
ARM64 macOS 14 Python 3	25m 45s
AMD64 Conda Python 3.10 Sphinx & Numpydoc	40m 54s
AMD64 Conda Python 3.12 without NumPy	15m 37s
AMD64 Windows 2022 Python 3.13	25m 50s
AMD64 Conda Python 3.13 Pandas latest	8m 14s
	2h 41m 30s

Provide feedback