[Cleanup] Remove unused SaveBackup in client.h (#3030)

# Notes
- This is unused and does nothing in both Perl and Lua.

zone/lua_client.h

@@ -34,7 +34,6 @@ public:
 	void Sit();
 	void Save();
 	void Save(int commit_now);
-	void SaveBackup();
 	bool Connected();
 	bool InZone();
 	void Kick();