Minor cleanup of webp decoding mode logic #3131

Job	Run time
linux (linux.12xlarge, cpu) / linux-job	58s
linux (linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job	59s
macos (macos-m1-stable) / macos-job	1m 41s
windows (windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job	37s
windows (windows.4xlarge, cpu) / windows-job	5m 38s
	9m 53s

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