cleanup player death event adventure logic (#10095)

There was a TODO left there regarding the translated death
message being used by plugins to identify the cause of death. This
should be mitigated now because the LegacyComponentSerializer default
implemenation uses our custom flattener which renders vanilla
translatable components to their English representation.

patches/api/PlayerDeathEvent-getItemsToKeep.patch

@@ -13,14 +13,14 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +++ b/src/main/java/org/bukkit/event/entity/PlayerDeathEvent.java
 @@ -0,0 +0,0 @@ public class PlayerDeathEvent extends EntityDeathEvent {
      }
-     // Paper end
+     // Paper end - adventure
  
 -    @Deprecated // Paper
      public PlayerDeathEvent(@NotNull final Player player, @NotNull final List<ItemStack> drops, final int droppedExp, @Nullable final String deathMessage) {
          this(player, drops, droppedExp, 0, deathMessage);
      }
 @@ -0,0 +0,0 @@ public class PlayerDeathEvent extends EntityDeathEvent {
-         this.adventure$deathMessage = deathMessage != null ? net.kyori.adventure.text.serializer.legacy.LegacyComponentSerializer.legacySection().deserialize(deathMessage) : null; // Paper
+         this.deathMessage = net.kyori.adventure.text.serializer.legacy.LegacyComponentSerializer.legacySection().deserializeOrNull(deathMessage); // Paper
      }
  
 +    @Deprecated // Paper