chore(synthetics): delete alpha module (#27441)

#27394 added back the synthetics module for CDK release v2.100.0 due to an issue with the peer dependencies that has since been fixed. Since synthetics is stabilized, the alpha module can be safely deleted now.

I downloaded `@aws-cdk/aws-synthetics-alpha` version `2.100.0-alpha.0` and confirmed that its `package.json` included the correct peer dependencies with the caret:

```json
  "peerDependencies": {
    "aws-cdk-lib": "^2.100.0",
    "constructs": "^10.0.0"
  },
```

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Author: kaizencc