bp: #38400
When the `Export` statement specifies the `delete_existing_files`
property, each `Outfile` statement generated by the `Export` will carry
this property. This causes each `Outfile` statement to delete existing
files, so only the result of the last Outfile statement will be
retained.
So, we add a rpc method which can delete existing files for `Export`
statement and the `Outfile` statements generated by the `Export` will
not carry `delete_existing_files` property any more.
## Proposed changes
Issue Number: close #xxx
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187461e2fd