Clear approval_merge_request_rule_source after merge

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A post issue after !8497 (merged)

The table approval_merge_request_rule_source is only used for open MRs, in order to find project level approvals_required count. For merged MR, it really does not matter. Overtime, as project rule gets deleted, the linkage would be nullified. But we can also remove the linkage when MR is merged, saving space.