remove unused port-forward PID captures

[delthas]

Summary

• Drop three _*_PF_PID=$! assignments in .github/scripts/end2end/setup-e2e-env.sh that are never read again — no wait, kill, or disown references the captured PIDs anywhere in the file.
• Pure cleanup, no behavior change. kubectl port-forward ... & already detaches into the background fine without the capture.

Removed

• _KAFKA_PF_PID
• _KAFKA_AUTH_PF_PID
• _PROM_PF_PID

Why

Flagged by @maeldonn on PR #2378 review for two new captures we'd added (_JAEGER_PF_PID, _CLOUDSERVER_INTERNAL_PF_PID); those were dropped in that PR. This one handles the three pre-existing ones.

Issue: ZENKO-5298

[bert-e]

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[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

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• 2 peers

[SylvainSenechal]

there was a problem with it, or is it a pure 3 lines removal cleanup 🤔