py-opencv4: dont require ffmpeg4

[percontation]

Description

There's no reason for the py-opencv4 subports to depend on ffmpeg4.

...In fact, it's probably the case that the python supports shouldn't depend on any of the super-port's library dependencies, but ffmpeg4 is the only annoying one (since, if the user selected a non-ffmpeg4 opencv4 variant, there isn't even any need for ffmpeg4 to be installed).

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 26.2 25C56 arm64
Xcode 26.2 17C52

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@mascguy for port opencv4.