Drop unused opencv BuildRequires

opencv has deliberately broken their C API as of 3.4 and completely
removed it in 4.0, meaning there is no way to enable OpenCV in ffmpeg
without completely rewriting the filter in C++.

https://github.com/opencv/opencv/wiki/ChangeLog#version400
https://ffmpeg.org/pipermail/ffmpeg-devel/2018-March/226872.html
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=915544

ffmpeg.spec

@@ -221,7 +221,6 @@ BuildRequires:  pkgconfig(ogg)
 BuildRequires:  pkgconfig(openal)
 BuildRequires:  pkgconfig(opencore-amrnb)
 BuildRequires:  pkgconfig(OpenCL)
-BuildRequires:  pkgconfig(opencv4)
 BuildRequires:  pkgconfig(opus)
 BuildRequires:  pkgconfig(rav1e)
 BuildRequires:  pkgconfig(rubberband)