Remove unused nav color constant (#22602)

frontend/src/metabase/nav/constants.js

-import { color, lighten } from "metabase/lib/colors";
-
-export const getDefaultSearchColor = () => lighten(color("nav"), 0.07);
-
 export const APP_BAR_HEIGHT = "52px";
 export const ADMIN_NAVBAR_HEIGHT = "65px";
 export const NAV_SIDEBAR_WIDTH = "324px";