Temporary fix to remove ffmpeg from build time (#1475)

* Fix cherry-pick conflict for 0.4.2 release

Author: fmassa

packaging/torchvision/meta.yaml

@@ -12,7 +12,6 @@ requirements:
   host:
     - python
     - setuptools
-    - av
     {{ environ.get('CONDA_PYTORCH_BUILD_CONSTRAINT') }}
     {{ environ.get('CONDA_CUDATOOLKIT_CONSTRAINT') }}
 
@@ -21,7 +20,6 @@ requirements:
     - pillow >=4.1.1
     - numpy >=1.11
     - six
-    - av
     {{ environ.get('CONDA_PYTORCH_CONSTRAINT') }}
     {{ environ.get('CONDA_CUDATOOLKIT_CONSTRAINT') }}