chore: remove gitleaks secrets scan workflow (#1184)

## Summary
- Removes the Gitleaks secrets scanning GitHub Actions workflow
(`.github/workflows/secrets-scan.yml`) because we did not receive
clarification from the vendor regarding the licensing question.

## Motivation
Tracked in JIRA:
[SVLS-8660](https://datadoghq.atlassian.net/browse/SVLS-8660)

## Test plan
- [ ] CI passes without the secrets scan workflow

🤖 Generated with [Claude Code](https://claude.com/claude-code)

[SVLS-8660]:
https://datadoghq.atlassian.net/browse/SVLS-8660?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: litianningdatadog