py-opencv4{,-devel}: remove explicit ffmpeg4 dependency

py-opencv4 does not directly depend on ffmpeg4, only opencv4.
Removing this extra dep matters because, if e.g. opencv4+ffmpeg8
were installed, installing py-opencv4 would install an extraneous
ffmpeg4 copy that isn't actually used.

graphics/opencv4-devel/Portfile

@@ -375,6 +375,8 @@ foreach python_branch ${python_branches} {
 
         depends_build-delete \
                     port:python${default_python_version}
+        depends_lib-delete \
+                    port:ffmpeg4
         depends_lib-append \
                     port:${name} \
                     port:py${python_version}-numpy \

graphics/opencv4/Portfile

@@ -428,6 +428,8 @@ foreach python_branch ${python_branches} {
 
         depends_build-delete \
                     port:python${default_python_version}
+        depends_lib-delete \
+                    port:ffmpeg4
         depends_lib-append \
                     port:${name} \
                     port:py${python_version}-numpy \