x11/plasma5-plasma-workspace: remove obsolete www/kf5-kjsembed dependency

This requirement was dropped three years ago upstream in [1] but was
missed in the ports tree.

[1] https://invent.kde.org/plasma/plasma-workspace/-/commit/6543190a87f745d841c64ef6c6d63b27a18386f4

Reported by: Rafael Sadowski <rafael@sizeofvoid.org>