Remove already exist assertion while renew ccr lease (#46009)

If a CCR lease is disappeared while we are renewing it, then we will
issue asyncAddRetentionLease to add that lease. And if
asyncAddRetentionLease takes longer than retentionLeaseRenewInterval,
then we can issue another asyncAddRetentionLease request. One of
asyncAddRetentionLease requests will fail with
RetentionLeaseAlreadyExistsException, hence trip the assertion.

Closes #45192

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardFollowTasksExecutor.java

@@ -44,7 +44,6 @@ import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.engine.CommitStats;
 import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.index.seqno.RetentionLeaseActions;
-import org.elasticsearch.index.seqno.RetentionLeaseAlreadyExistsException;
 import org.elasticsearch.index.seqno.RetentionLeaseNotFoundException;
 import org.elasticsearch.index.seqno.SeqNoStats;
 import org.elasticsearch.index.shard.ShardId;
@@ -439,7 +438,6 @@ public class ShardFollowTasksExecutor extends PersistentTasksExecutor<ShardFollo
                                                      * going on. Log it, and renew again after another renew interval has passed.
                                                      */
                                                     final Throwable innerCause = ExceptionsHelper.unwrapCause(inner);
-                                                    assert innerCause instanceof RetentionLeaseAlreadyExistsException == false;
                                                     logRetentionLeaseFailure(retentionLeaseId, innerCause);
                                                 }));
                             } else {