applets/taskmanager: Drop pipewire window thumbnail opacity animation

It causes missing window thumbnails. It's not precisely clear what is
going on, the bug is very difficult to debug.

The original purpose was to make things more eyecandy because window
thumbnails will be presented slightly later. But one could argue that
the animation only makes the delay more noticeable. The window thumbnail
might be missing for a few frames. When the opacity animation plays,
it effectively extends how long the background behind the thumbnail is
visible.

(cherry picked from commit a63a802f)