seat: Don't forget to destroy touch points on touch up
Otherwise the number of touch points goes up constantly and d'n'd via touch can't work as validation always fails.
Fixes 75ecba44 ("seat: add serials to touch up events")
Might be worth to backport this to 0.18.x, can do a MR if that looks sane.
Signed-off-by: Guido Günther agx@sigxcpu.org