🚮 Sweep experiments older than 2021-03-02

Sweep experiments last flipped globally up to 2021-03-02:

- Remove `alp`: 0

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### Cleanup issues

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

- [ ] `alp`: ampproject#4005

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### ⚠️ Javascript source files require intervention

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

- [ ] `src/impression.js`
- [ ] `test/unit/test-impression.js`

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

Author: alanorozco