[BP] MB-58961: Remove seqno checks for backfill from AS:registerCursor

Checks originally introduced in MB-22960.
In the current code base those checks are already enforced in
CM::registerCursorBySeqno(), which is were we determine whether a
backfill is needed or not.

That area is in general well covered by unit tests. Specifically for
MB-22960 we have the following original tests that guard this change:

- MB22960_cursor_dropping_data_loss
- MB25056_do_not_set_pendingBackfill_to_true
- MB25056_backfill_not_required

Change-Id: I573059d2d05701e2024b890b1d84d708768e9833
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/198871
Reviewed-by: Trond Norbye <[email protected]>
Well-Formed: Restriction Checker
Tested-by: Paolo Cocchi <[email protected]>

Author: paolococchi

engines/ep/src/dcp/active_stream.cc

@@ -256,25 +256,7 @@ void ActiveStream::registerCursor(CheckpointManager& chkptmgr,
         CursorRegResult result = chkptmgr.registerCursorBySeqno(
                 name_, lastProcessedSeqno, CheckpointCursor::Droppable::Yes);
 
-        /*
-         * MB-22960:  Due to cursor dropping we re-register the replication
-         * cursor only during backfill when we mark the disk snapshot.  However
-         * by this point it is possible that the CheckpointManager no longer
-         * contains the next sequence number the replication stream requires
-         * (i.e. next one after the backfill seqnos).
-         *
-         * To avoid this data loss when we register the cursor we check to see
-         * if the result is greater than the lastProcessedSeqno + 1.
-         * If so we know we may have missed some items and may need to perform
-         * another backfill.
-         *
-         * We actually only need to do another backfill if the result is greater
-         * than the lastProcessedSeqno + 1 and registerCursorBySeqno returns
-         * true, indicating that the resulting seqno starts with the first item
-         * on a checkpoint.
-         */
-        const uint64_t nextRequiredSeqno = lastProcessedSeqno + 1;
-        if (result.seqno > nextRequiredSeqno && result.tryBackfill) {
+        if (result.tryBackfill) {
             pendingBackfill = true;
         }
         curChkSeqno = result.seqno;