applets/taskmanager: Remove the icon size option from vertical taskmanager

This is a classic "too many sources of truth" problem, because the icon size is
affected by multiple factors:

1. The panel thickness and margins
2. Whether it's a vertical or horizontal panel
3. The size set in the Icons KCM (which is actually a maximum size)
4. The size set in the settings window (which is capped to the maximum size set
   in the icons KCM)

There are so many variables here that in practice it is practically impossible to
actually adjust the size to your liking. Removing the setting specific to the Task
Manager here makes icon sizing simpler and more predictable, and unifies the
behavior with the horizontal version.