Cleanup `onConcurrentShardSince` in `sharded_backup_restore.js`

The sharded_backup_restore.js test is simulating a restore of a sharded cluster, resetting the history when the chunks are moved on a new cluster:

            assert.commandWorked(configDb.chunks.update(

                {shard: sourceShardName}, {$set: {shard: destShardName, history: []}}));

Since the field added under SERVER-71615 always need to be consistent with the first element of the history, the test needs to be changed in order to also clear up that field.