requestbatcher: nil unused slice entries in batchQueue heap

This change nils out unused entries in the batchQueue's slice when the
slice is truncated to avoid retaining references to batches of requests
and preventing them from being GCed.

The change also removes a redundant update of the batchQueue's byRange
map. The `delete` was redundant because `heap.Remove` already calls `Pop`.

pkg/internal/client/requestbatcher/batcher.go

@@ -639,7 +639,6 @@ func (q *batchQueue) get(id roachpb.RangeID) (*batch, bool) {
 }
 
 func (q *batchQueue) remove(ba *batch) {
-	delete(q.byRange, ba.rangeID())
 	heap.Remove(q, ba.idx)
 }
 
@@ -678,6 +677,7 @@ func (q *batchQueue) Push(v interface{}) {
 
 func (q *batchQueue) Pop() interface{} {
 	ba := q.batches[len(q.batches)-1]
+	q.batches[len(q.batches)-1] = nil // for GC
 	q.batches = q.batches[:len(q.batches)-1]
 	delete(q.byRange, ba.rangeID())
 	ba.idx = -1