Remove unused code from the KnownAssignedTransactionIdes submodule

[Quan Zongliang]

Hi,
In the KnownAssignedTransactionIdes sub-module, two lines of unused code 
were found in a previous change.

--
Quan Zongliang
CPUG
diff --git a/src/backend/storage/ipc/procarray.c 
b/src/backend/storage/ipc/procarray.c
index b448533564..99ce8c752a 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -3363,8 +3363,6 @@ RecordKnownAssignedTransactionIds(TransactionId xid)
 
                /* ShmemVariableCache->nextFullXid must be beyond any observed 
xid */
                AdvanceNextFullTransactionIdPastXid(latestObservedXid);
-               next_expected_xid = latestObservedXid;
-               TransactionIdAdvance(next_expected_xid);
        }
 }