Remove need of redundant argument ffmpeg

[ashwani-rathee]

• get_camera_devices doesn't seem to be using the ffmpeg argument

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (45801ef) 79.40% compared to head (993ea86) 79.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #408 +/- ## ======================================= Coverage 79.40% 79.40% ======================================= Files 10 10 Lines 1282 1282 ======================================= Hits 1018 1018 Misses 264 264 ``` | [Files](https://app.codecov.io/gh/JuliaIO/VideoIO.jl/pull/408?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/avio.jl](https://app.codecov.io/gh/JuliaIO/VideoIO.jl/pull/408?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2F2aW8uamw=) | `76.38% <0.00%> (ø)` | | | [src/VideoIO.jl](https://app.codecov.io/gh/JuliaIO/VideoIO.jl/pull/408?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL1ZpZGVvSU8uamw=) | `42.18% <0.00%> (ø)` | |

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[sjkelly]

Thanks!