delete unused code - CodeScanningAlert (#727)

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This file slipped through the cracks in the Secret Scanning Alert
migration PR recently. This should have been deleted as it isn't used
anywhere. I think there is a future PR coming that will migrate Code
Scanning Alerts, but until that time we shouldn't have dead code in
main.

Related to #536 

- [x] Did you write/update appropriate tests
- [x] Release notes updated (if appropriate)
- [x] Appropriate logging output
- [x] Issue linked
- [x] Docs updated (or issue created)

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Author: dylan-smith