[heal_unit]: clear the animation after playing it (#3526)

Fixes another case of unit halo remaining after death.

(cherry-picked from commit 3541629)

data/lua/wml/heal_unit.lua

@@ -44,6 +44,7 @@ function wesnoth.wml_actions.heal_unit(cfg)
 				animator:add(healers[1], 'healing', 'hit', {value = heal_amount})
 			end
 			animator:run()
+			animator:clear()
 		end
 
 		u.hitpoints = new_hitpoints