Minor cleanup of webp decoding mode logic #3318

Job	Run time
unittests-extended	0s
onnx / linux-job	55s
unittests-linux (3.10, linux.12xlarge, cpu) / linux-job	56s
unittests-linux (3.11, linux.12xlarge, cpu) / linux-job	47s
unittests-linux (3.12, linux.12xlarge, cpu) / linux-job	44s
unittests-linux (3.9, linux.12xlarge, cpu) / linux-job	54s
unittests-linux (3.9, linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job	1m 3s
unittests-macos (3.10, macos-m1-stable) / macos-job	1m 21s
unittests-macos (3.11, macos-m1-stable) / macos-job	1m 14s
unittests-macos (3.12, macos-m1-stable) / macos-job	1m 16s
unittests-macos (3.9, macos-m1-stable) / macos-job	1m 15s
unittests-windows (3.10, windows.4xlarge, cpu) / windows-job	5m 20s
unittests-windows (3.11, windows.4xlarge, cpu) / windows-job	6m 0s
unittests-windows (3.9, windows.4xlarge, cpu) / windows-job	5m 57s
unittests-windows (3.12, windows.4xlarge, cpu) / windows-job	5m 37s
unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job	51s
	34m 10s

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