Revert: Only expose used CSS variables (#16403)

This reverts #16211

We found some unexpected interactions with using `@apply` and CSS
variables in multi-root setups like CSS modules or Vue inline `<style>`
blocks that were broken due to that change. We plan to re-enable this
soon and include a proper fix for those scenarios.

## Test plan

- Updated snapshots
- Tested using the CLI in a new project:
<img width="1523" alt="Screenshot 2025-02-10 at 13 08 42"
src="https://github.com/user-attachments/assets/defe0858-adb3-4d61-9d2c-87166558fd68"
/>

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

integrations/cli/index.test.ts

@@ -1289,7 +1289,9 @@ test(
     expect(await fs.dumpFiles('./dist/*.css')).toMatchInlineSnapshot(`
       "
       --- ./dist/out.css ---
-      <EMPTY>
+      :root, :host {
+        --color-blue-500: blue;
+      }
       "
     `)