libvmaf filter attempts to load deprecated model file

[veikk0]

Summary of the bug:

When built with libvmaf v2.0.0 or later, attempting to invoke the libvmaf filter with FFmpeg (with the vmaf_v0.6.1.pkl file in the correct location) will fail with the following error:

libvmaf INFO `compute_vmaf()` is deprecated and will be removed in a future libvmaf version
libvmaf WARNING could not read model from path: "/usr/local/share/model/vmaf_v0.6.1.pkl"
libvmaf WARNING pkl model files have been deprecated, use json
libvmaf ERROR problem loading model file: /usr/local/share/model/vmaf_v0.6.1.pkl
[Parsed_libvmaf_0 @ 0x55f2b8286f80] libvmaf encountered an error, check log for details
Error while filtering: Invalid argument
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #1:0

Currently the default value for the model_path parameter in the libvmaf filter is "/usr/local/share/model/vmaf_v0.6.1.pkl". However, as the error message implies, support for .pkl models was removed in ​VMAF 2.0.0 and .json models are used instead.

In addition, v2.0.0 added support for built-in models (-Dbuilt_in_models=true build option, enabled by default). So having a default model path probably wouldn't even be required.

How to reproduce:

ffmpeg -r 30 -i encoded.webm -r 30 -i original.mp4 -an -sn -lavfi libvmaf -f null -

Version information:

ffmpeg version N-100946-ge6254d5 Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 10 (Ubuntu 10.1.0-2ubuntu1~18.04)

[Gyan]

See last comment in #9047