Remove invalid distinct key from postfix very-slow user-enum scenario

[Copilot]

Description

The review feedback flagged distinct: evt.Meta.sasl_username as non-functional for this scenario. This update applies the requested change exactly by removing distinct from the melite/postfix-very-slow-bf_user-enum definition.

• Scope
  • Updated only scenarios/melite/postfix-very-slow-bf.yaml.
• Change
  • Removed distinct: evt.Meta.sasl_username from the user-enum variant.
• Before/After

  # before
  groupby: evt.Meta.source_ip
  distinct: evt.Meta.sasl_username
  leakspeed: "4h"
  
  # after
  groupby: evt.Meta.source_ip
  leakspeed: "4h"

Checklist

• I have read the contributing guide
• I have tested my changes locally
• For new parsers or scenarios, tests have been added
• I have run the hub linter and no issues were reported (see contributing guide)
• Automated tests are passing
• AI was used to generate any/all content of this PR