Remove global checkpoint assertion in peer recovery (elastic#41987)

If remote recovery copies an index commit which has gaps in sequence
numbers to a follower; then these assertions (introduced in elastic#40823)
don't hold for follower replicas.

Closes elastic#41037

server/src/main/java/org/elasticsearch/indices/recovery/RecoveryTarget.java

@@ -26,7 +26,6 @@
 import org.elasticsearch.Assertions;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.ExceptionsHelper;
-import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.common.UUIDs;
@@ -39,7 +38,6 @@
 import org.elasticsearch.index.mapper.MapperException;
 import org.elasticsearch.index.seqno.ReplicationTracker;
 import org.elasticsearch.index.seqno.RetentionLeases;
-import org.elasticsearch.index.seqno.SequenceNumbers;
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.index.shard.IndexShardNotRecoveringException;
 import org.elasticsearch.index.shard.IndexShardState;
@@ -288,9 +286,6 @@ public void prepareForTranslogOperations(boolean fileBasedRecovery, int totalTra
         ActionListener.completeWith(listener, () -> {
             state().getTranslog().totalOperations(totalTranslogOps);
             indexShard().openEngineAndSkipTranslogRecovery();
-            assert indexShard.getGlobalCheckpoint() >= indexShard.seqNoStats().getMaxSeqNo() ||
-                indexShard.indexSettings().getIndexVersionCreated().before(Version.V_7_2_0)
-                : "global checkpoint is not initialized [" + indexShard.seqNoStats() + "]";
             return null;
         });
     }
@@ -396,9 +391,6 @@ public void cleanFiles(int totalTranslogOps, long globalCheckpoint, Store.Metada
         store.incRef();
         try {
             store.cleanupAndVerify("recovery CleanFilesRequestHandler", sourceMetaData);
-            assert globalCheckpoint >= Long.parseLong(sourceMetaData.getCommitUserData().get(SequenceNumbers.MAX_SEQ_NO))
-                || indexShard.indexSettings().getIndexVersionCreated().before(Version.V_7_2_0) :
-                "invalid global checkpoint[" + globalCheckpoint + "] source_meta_data [" + sourceMetaData.getCommitUserData() + "]";
             final String translogUUID = Translog.createEmptyTranslog(
                 indexShard.shardPath().resolveTranslog(), globalCheckpoint, shardId, indexShard.getPendingPrimaryTerm());
             store.associateIndexWithNewTranslog(translogUUID);