webdav: remove unnecessary "n.byExpiryIndex = -1".

These occur after a heap.Remove call, and heap.Remove calls
byExpiry.Pop, which already sets n.byExpiryIndex to -1.

The memLS.consistent method in lock_test.go checks that the expiry
fields are all consistent.

Change-Id: I09f5d02999e4635f15d1723f89a86b951ed95950
Reviewed-on: https://go-review.googlesource.com/3563
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Nick Cooper <nmvc@google.com>

webdav/lock.go

@@ -193,7 +193,6 @@ func (m *memLS) Refresh(now time.Time, token string, duration time.Duration) (Lo
 	}
 	if n.byExpiryIndex >= 0 {
 		heap.Remove(&m.byExpiry, n.byExpiryIndex)
-		n.byExpiryIndex = -1
 	}
 	n.details.Duration = duration
 	if n.details.Duration >= 0 {
@@ -277,7 +276,6 @@ func (m *memLS) remove(n *memLSNode) {
 	})
 	if n.byExpiryIndex >= 0 {
 		heap.Remove(&m.byExpiry, n.byExpiryIndex)
-		n.byExpiryIndex = -1
 	}
 }