Preferences: delete any MP passwords saved with the legacy MP method

Even though we switched to encrypted local password storage a while back, any passwords
saved with the old plaintext method remained in player's preferences file and were loaded
by the game where there's no reason they couldn't have been directly accessed.

src/preferences/game.cpp

@@ -88,6 +88,10 @@ manager::manager() :
 		preferences::erase("mp_countdown_action_bonus");
 	}
 
+	// We save the password encrypted now. Erase any saved passwords in the prefs file.
+	preferences::erase("password");
+	preferences::erase("password_is_wrapped");
+
 	/*
 	completed_campaigns = "A,B,C"
 	[completed_campaigns]