Backport of allocwatcher: don't destroy local allocdir after migration into release/1.6.x #18132
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Backport
This PR is auto-generated from #18108 to be assessed for backporting due to the inclusion of the label backport/1.6.x.
The below text is copied from the body of the original PR.
When ephemeral disks are migrated from an allocation on the same node, allocation logs for the previous allocation are lost.
There are two workflows for the best-effort attempt to migrate the allocation data between the old and new allocations. For previous allocations on other clients (the "remote" workflow), we create a local allocdir and download the data from the previous client into it. That data is then moved into the new allocdir and we delete the allocdir of the previous alloc.
For "local" previous allocations we don't need to create an extra directory for the previous allocation and instead move the files directly from one to the other. But we still delete the old allocdir entirely, which includes all the logs!
There doesn't seem to be any reason to destroy the local previous allocdir, as the usual client garbage collection should destroy it later on when needed. By not deleting it, the previous allocation's logs are still available for the user to read.
Fixes: #18034