Fix crash when opening object picker window.

SELGW = ScenarioEditorLandscapeGenerationWindow
Crash is triggered by the following:
1. SELGW has a OnPlaceObject active (e.g. raise/lower land).
2. SELGW button is clicked to open BuildObjectWindow
3. BuildObjectWindow constructor calls ResetObjectToPlace
4: SELGW::OnPlaceObjectAbort is called
5: DeleteWindowById(WC_BUILD_OBJECT, 0) is called
6: BuildObjectWindow is freed whilst its constructor is executing
7: Crash occurs either through access to this->vscroll
   or during the dynamic_cast to get widget: WID_BO_SELECT_MATRIX.

The use-after-free was caught by AddressSanitizer.

src/terraform_gui.cpp

@@ -739,9 +739,6 @@ struct ScenarioEditorLandscapeGenerationWindow : Window {
 	{
 		this->RaiseButtons();
 		this->SetDirty();
-		DeleteWindowById(WC_BUILD_OBJECT, 0);
-		DeleteWindowById(WC_BUILD_HOUSE, 0);
-		DeleteWindowById(WC_SELECT_STATION, 0);
 	}
 
 	static HotkeyList hotkeys;