kvclient: remove no longer valid assertion about batch splitting

This commit removes a fatal assertion that batch with MaxSpanRequestKeys
or TargetBytes set doesn't need to be split. This assertion has been in
place for many years, but it no longer holds true in some cases with
buffered writes (namely when we buffered some writes, but then they were
disabled, so we need to flush the buffer together with the next batch:
that next batch can fail the assertion).

Release note: None

Author: yuzefovich

pkg/kv/kvclient/kvcoord/dist_sender.go

@@ -1222,12 +1222,6 @@ func (ds *DistSender) Send(
 		splitET = true
 	}
 	parts := splitBatchAndCheckForRefreshSpans(ba, splitET)
-	if len(parts) > 1 && (ba.MaxSpanRequestKeys != 0 || ba.TargetBytes != 0) {
-		// We already verified above that the batch contains only scan requests of the same type.
-		// Such a batch should never need splitting.
-		log.Fatalf(ctx, "batch with MaxSpanRequestKeys=%d, TargetBytes=%d needs splitting",
-			redact.Safe(ba.MaxSpanRequestKeys), redact.Safe(ba.TargetBytes))
-	}
 	var singleRplChunk [1]*kvpb.BatchResponse
 	rplChunks := singleRplChunk[:0:1]