Remove flaky assertion in FullClusterRestartIT.testRecovery (opensearch-project#20747)

The test asserts that after a full cluster restart upgrade, at least one
old version Lucene segment must still exist on the primary. This is not
guaranteed because background merges can merge all old version segments
into new segments before the assertion runs.

The remaining assertions still validate that translog replay occurred
and produced at least one new-version segment on the primary.

Signed-off-by: Andrew Ross <andrross@amazon.com>

Author: andrross

qa/full-cluster-restart/src/test/java/org/opensearch/upgrades/FullClusterRestartIT.java

@@ -793,7 +793,6 @@ public void testRecovery() throws Exception {
                 }
                 assertNotEquals("expected at least 1 current segment after translog recovery. segments:\n" + segmentsResponse,
                     0, numCurrentVersion);
-                assertNotEquals("expected at least 1 old segment. segments:\n" + segmentsResponse, 0, numBwcVersion);
             }
         }
     }