kvclient: remove ReturnOnRangeBoundary request option

This was added in #70763 with the intention of using it for the
ExportRequests sent by backup processors.

In #114268 we went with a different approach and this feature has not seen
subsequent use.

BatchRequest options that may result in partial batch results require
special thinking/handling at multiple points in dist_sender, so it seems
sensible to remove this option if it is unused.

Epic: none
Release note: None

Author: stevendanna

pkg/kv/kvclient/kvcoord/dist_sender.go

@@ -2040,13 +2040,6 @@ func (ds *DistSender) divideAndSendBatchToRanges(
 						return
 					}
 				}
-				// If we hit a range boundary, return a partial result if requested. We
-				// do this after checking the limits, so that they take precedence.
-				if ba.Header.ReturnOnRangeBoundary && replyKeys > 0 && !lastRange {
-					couldHaveSkippedResponses = true
-					resumeReason = kvpb.RESUME_RANGE_BOUNDARY
-					return
-				}
 			}
 		}