lease: Clean-up old TODO around lease map and priority queue

This TODO was addressed in #9418.

lease/lessor.go

@@ -123,12 +123,6 @@ type lessor struct {
 	// demotec will be closed if the lessor is demoted.
 	demotec chan struct{}
 
-	// TODO: probably this should be a heap with a secondary
-	// id index.
-	// Now it is O(N) to loop over the leases to find expired ones.
-	// We want to make Grant, Revoke, and findExpiredLeases all O(logN) and
-	// Renew O(1).
-	// findExpiredLeases and Renew should be the most frequent operations.
 	leaseMap  map[LeaseID]*Lease
 	leaseHeap LeaseQueue
 	itemMap   map[LeaseItem]LeaseID