Remove need of redundant argument ffmpeg #248

Job	Run time
test (ubuntu-latest, x64, 1.6)	4m 50s
test (ubuntu-latest, x64, 1)	4m 41s
test (ubuntu-latest, x64, 1.9-nightly)	5m 26s
test (ubuntu-latest, x64, nightly)	5m 4s
test (ubuntu-latest, x86, 1.6)	5m 28s
test (ubuntu-latest, x86, 1)	4m 36s
test (ubuntu-latest, x86, 1.9-nightly)	6m 18s
test (ubuntu-latest, x86, nightly)	6m 39s
test (windows-latest, x64, 1.6)	14m 28s
test (windows-latest, x64, 1)	7m 36s
test (windows-latest, x64, 1.9-nightly)	6m 59s
test (windows-latest, x64, nightly)	8m 42s
test (windows-latest, x86, 1.6)	14m 54s
test (windows-latest, x86, 1)	7m 26s
test (windows-latest, x86, 1.9-nightly)	8m 13s
test (windows-latest, x86, nightly)	8m 24s
test (macOS-latest, x64, 1.6)	6m 32s
test (macOS-latest, x64, 1)	5m 55s
test (macOS-latest, x64, 1.9-nightly)	6m 20s
test (macOS-latest, x64, nightly)	10m 8s
	2h 28m 39s

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