x11/plasma5-plasma-workspace: drop obsolete dependency on webkit

This was removed in [1] and [2] quite a while ago.

[1] https://invent.kde.org/plasma/plasma-workspace/-/commit/9c1d4f889d32a1b54938709c8c5dd9ca975532a2
[2] https://invent.kde.org/plasma/plasma-workspace/-/commit/6aedc9534f76f9fb0d5d9560f81a704bda1e8df7