[fix](Export) fix a memory leak in the FE because of the ExportJob

[BePPPower]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:
When an Export job has finished or been cancelled, ExportMgr will maintain a reference of the ExportJob object. Therefore, we need to clear these no longer used objects; otherwise it will cause a memory leak in the FE.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason: It can not be figured out in neither Unit Test nor Regression test.

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
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4. Which code was refactored and why was this part of the code refactored?
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[BePPPower]

run buildall

[morningman]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[kaka11chen]

LGTM