opencv4: drop ffmpeg_4 pin

pkgs/top-level/all-packages.nix

···
     inherit (darwin.apple_sdk.frameworks)
       AVFoundation Cocoa VideoDecodeAcceleration CoreMedia MediaToolbox Accelerate;
     pythonPackages = python3Packages;
-    ffmpeg = ffmpeg_4;
   };
 
   opencv4WithoutCuda = opencv4.override {