[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

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
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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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• [] I have added tests that prove my fix is effective or that my feature works
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[EmmyMiao87]

LGTM