verbosely ensure items cannot be duplicated when keeping inventory on death

Seemingly there's duplication issues on some servers (and apparently even when only PlotSquared is installed). Likely a Spigot issue, but there's nothing to be done for Spigot on 1.13/14/15

Bukkit/src/main/java/com/plotsquared/bukkit/listener/PlayerEventListener.java

@@ -1509,6 +1509,7 @@ public class PlayerEventListener extends PlotListener implements Listener {
             if (plot.getFlag(KeepInventoryFlag.class)) {
                 plot.debug(event.getEntity().getName()
                     + " kept their inventory because of keep-inventory = true");
+                event.getDrops().clear();
                 event.setKeepInventory(true);
             }
         }