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

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

zone/client.cpp

@@ -743,9 +743,6 @@ bool Client::Save(uint8 iCommitNow) {
 	return true;
 }
 
-void Client::SaveBackup() {
-}
-
 CLIENTPACKET::CLIENTPACKET()
 {
 	app = nullptr;