Skip unreferenced files pruning after restore for searchable snapshots shards #68845
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When a shard is restored from a snapshot there is an after
restore step during which files on disk that are not referenced
anymore by the restored segments infos file are deleted from
disk. The deletion is executed using the
Lucene.pruneUnreferencedFiles() method which opens a new
IndexWriter and closes it in order to kick off Lucene's internal
index files deleter.
This step is not necessary for searchable snapshot shards
which use the snapshot store type because such shards
report a list of files coming from a shard snapshot which
never contain unreferenced files. Skipping this step avoids
extra unnecessary work when mounting a searchable
snapshot index.
Backport of #68821