SERVER-42581 Remove set FCV in shard_identity_rollback.js

Author: Misha Tyulenev

jstests/sharding/shard_identity_rollback.js

@@ -19,10 +19,6 @@ var priConn = replTest.getPrimary();
 var secondaries = replTest.getSecondaries();
 var configConnStr = st.configRS.getURL();
 
-// Shards start in FCV 4.0 until a config server reaches out to them. This causes storage to
-// shutdown with 4.0 compatible files, requiring rollback via refetch.
-priConn.adminCommand({setFeatureCompatibilityVersion: "4.0"});
-
 // Wait for the secondaries to have the latest oplog entries before stopping the fetcher to
 // avoid the situation where one of the secondaries will not have an overlapping oplog with
 // the other nodes once the primary is killed.