Revert "Remove workaround in translog rest test (#27530)"

Even though the option wait_for_no_initializing_shards is backported to
6.2, we should not remove this workaround on 6.3 because this rest test
will be run in a mixed cluster in which the
wait_for_no_initializing_shards may not be available.

This reverts commit 7aea8b5.

rest-api-spec/src/main/resources/rest-api-spec/test/indices.stats/20_translog.yml

@@ -3,9 +3,12 @@ setup:
   - do:
       indices.create:
           index: test
-  - do:
-      cluster.health:
-        wait_for_no_initializing_shards: true
+          body:
+           settings:
+             index:
+               # initializing replicas maintain the translog causing the test to fail.
+               # remove once https://github.com/elastic/elasticsearch/issues/25623 is fixed.
+               number_of_replicas: 0
 
 ---
 "Translog retention":