fix(sdam): topology no longer causes close event

The topology was emitting both a "topologyClosed" and "close"
event which are bubbled up to the MongoClient. This was causing
two "close" events to be emitted from the MongoClient when the
client is closed.

NODE-3219

lib/core/sdam/topology.js

@@ -359,7 +359,6 @@ class Topology extends EventEmitter {
           this.emit('topologyClosed', new events.TopologyClosedEvent(this.s.id));
 
           stateTransition(this, STATE_CLOSED);
-          this.emit('close');
 
           if (typeof callback === 'function') {
             callback(err);