allocwatcher: don't destroy local allocdir after migration

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

.changelog/18108.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+migration: Fixed a bug where previous alloc logs were destroyed when migrating ephemeral_disk on the same client
+```

client/allocwatcher/alloc_watcher.go

@@ -279,15 +279,7 @@ func (p *localPrevAlloc) Migrate(ctx context.Context, dest *allocdir.AllocDir) e
 
 	p.logger.Debug("copying previous alloc")
 
-	moveErr := dest.Move(p.prevAllocDir, p.tasks)
-
-	// Always cleanup previous alloc
-	if err := p.prevAllocDir.Destroy(); err != nil {
-		p.logger.Error("error destroying alloc dir",
-			"error", err, "previous_alloc_dir", p.prevAllocDir.AllocDir)
-	}
-
-	return moveErr
+	return dest.Move(p.prevAllocDir, p.tasks)
 }
 
 // remotePrevAlloc is a prevAllocWatcher for previous allocations on remote