Delete unused `metadata.js` file (#15037)

frontend/test/__support__/metadata.js

-// TODO: consolidate with sample_dataset_fixture.js once we get get cypress to load metabase-lib
-// TODO: dump sample_dataset_fixture.json from same snapshot as used in Cypress tests
-
-export const SAMPLE_DATASET = {
-  id: 1,
-};
-
-export const PRODUCTS = {
-  CATEGORY: {
-    id: 6,
-  },
-};