Remove README section about video decoding (#9172)

Author: NicolasHug

README.md

@@ -61,19 +61,6 @@ Torchvision currently supports the following image backends:
 
 Read more in in our [docs](https://pytorch.org/vision/stable/transforms.html).
 
-## [UNSTABLE] Video Backend
-
-Torchvision currently supports the following video backends:
-
-- [pyav](https://github.com/PyAV-Org/PyAV) (default) - Pythonic binding for ffmpeg libraries.
-- video_reader - This needs ffmpeg to be installed and torchvision to be built from source. There shouldn't be any
-  conflicting version of ffmpeg installed. Currently, this is only supported on Linux.
-
-```
-conda install -c conda-forge 'ffmpeg<4.3'
-pip install . -v --no-build-isolation
-```
-
 # Using the models on C++
 
 Refer to [example/cpp](https://github.com/pytorch/vision/tree/main/examples/cpp).