🚮 Sweep experiments older than 2021-08-10

Sweep experiments last flipped globally up to 2021-08-10:

- (2021-03-05, 77127c7) `disable-a4a-non-sd`: 1
- (2021-03-23, 1115f6c) `amp-cid-backup`: 1
- (2021-07-22, 7b1df94) `amp-story-page-attachment-ui-v2`: 1

---

### Cleanup issues

Close these once they've been addressed and this PR has been merged:

- [ ] `amp-cid-backup`: #29484

---

### ⚠️ Javascript source files require intervention

The following may contain errors and/or require intervention to remove superfluous conditionals:

- [ ] `extensions/amp-a4a/0.1/amp-a4a.js`
- [ ] `extensions/amp-a4a/0.1/test/test-amp-a4a.js`
- [ ] `src/service/cid-impl.js`

Refer to the removal guide for [suggestions on handling these modified Javascript files.](https://github.com/ampproject/amphtml/blob/main/build-system/tasks/sweep-experiments/README.md#followup)