#761 Fix removal and restoration of primary permission group

- Improve how a player is being switched between permission groups (add new group before removing old one)
- Remove group handling logic from LimboCache: AuthGroupHandler is now solely responsible for changing the player's permission group

src/main/java/fr/xephi/authme/service/CommonService.java

@@ -101,10 +101,9 @@ public class CommonService {
      *
      * @param player the player to process
      * @param group the group to add the player to
-     * @return true on success, false otherwise
      */
-    public boolean setGroup(Player player, AuthGroupType group) {
-        return authGroupHandler.setGroup(player, group);
+    public void setGroup(Player player, AuthGroupType group) {
+        authGroupHandler.setGroup(player, group);
     }
 
 }