Remove old regex-style Slack token DLP rule

[randypargman]

Summary

• Removes dlp_slack_token.yml, the old regex-based Slack token detection rule
• The new recognizer-based rule for Slack access tokens supersedes this

Test plan

• Confirm new Slack access token recognizer rule is active and covers the same detection surface

[github-actions]

Shared Samples Sync - Action Required

This PR was not automatically synced to shared-samples because the author is not a member of the sublime-security organization.

To enable syncing, an organization member can comment /update-shared-samples on this PR.

Once triggered, the rules will be synced on the next scheduled run (every 10 minutes).

[Jummyberry]

Verified that beta_dlp_slack_access_token (ML recognizer-based rule) is active and covers the same detection surface as the old regex rule. Ran the old regex pattern as a hunt against shared samples 0 hits, confirming no active detection is being lost with this removal. Approving