[animate_unit]: clear the animation after playing it

This avoids a reference to the unit from being leaked, which prevented the
unit's halo (if any) from disappearing when the unit died.

Fixes #3509. Closes #3520.

(cherry-picked from commit e63bd35f9d)

data/lua/wml/animate_unit.lua

@@ -110,4 +110,5 @@ function wesnoth.wml_actions.animate_unit(cfg)
 	local anim = wesnoth.create_animator()
 	add_animation(anim, cfg)
 	anim:run()
+	anim:clear()
 end