Fix name mapping not being cleared on switch

This fixes a bug where tablist would be messed up for 1.7 clients due to
an internal misconception.

proxy/src/main/java/com/velocitypowered/proxy/tablist/VelocityTabListLegacy.java

@@ -52,6 +52,7 @@ public class VelocityTabListLegacy extends VelocityTabList {
           Collections.singletonList(PlayerListItem.Item.from(value))));
     }
     entries.clear();
+    nameMapping.clear();
   }
 
   @Override