[Bug] fix finished load jobs cost too much heap

[qidaye]

Proposed changes

Since the plan is retained in the task, if the task is not cleaned up, the memory usage will be too large caused Memory leak or OOM.
When load job finished, there is no need to hold the tasks which are the biggest memory consumers.
Fixed #4992

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• [] Code refactor (Modify the code structure, format the code, etc...)

Checklist

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• I have create an issue on (Fix #ISSUE), and have described the bug/feature there in detail
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[EmmyMiao87]

LGTM