Fix: Open scenario editor date query once. (#10050)

Clearing `_left_button_clicked` causes the button callback to be fired
constantly while the mouse button is held.

src/toolbar_gui.cpp

@@ -1152,7 +1152,6 @@ static CallBackFunction ToolbarScenDatePanel(Window *w)
 {
 	SetDParam(0, _settings_game.game_creation.starting_year);
 	ShowQueryString(STR_JUST_INT, STR_MAPGEN_START_DATE_QUERY_CAPT, 8, w, CS_NUMERAL, QSF_ENABLE_DEFAULT);
-	_left_button_clicked = false;
 	return CBF_NONE;
 }