If the pointer capability is added, pointer swipe and pinch gestures
will be created. However, if the pointer capability is removed, the
gesture objects won't be destroyed.
If the pointer capability is removed and added several times in a row,
for example due to plugging and unplugging physical mouse, this can lead
to leaking the old gesture objects.
In order to prevent that, this change makes the seat destroy swipe and
pinch gestures when the pointer capability is withdrawn.
