fix(sessions): never send endSessions from a ClientSession

Sessions are meant to be cached, and the `endSessions` command is
only meant to be used as a way to signal to the server that cached
sessions will no longer be used (due to the client closing). We
were sending too many `endSessions` commands, now we just send the
one.

NODE-1418

lib/sessions.js

@@ -78,7 +78,6 @@ class ClientSession extends EventEmitter {
    * Ends this session on the server
    *
    * @param {Object} [options] Optional settings
-   * @param {Boolean} [options.skipCommand] Skip sending the actual endSessions command to the server
    * @param {Function} [callback] Optional callback for completion of this operation
    */
   endSession(options, callback) {
@@ -94,11 +93,6 @@ class ClientSession extends EventEmitter {
       this.abortTransaction(); // pass in callback?
     }
 
-    if (!options.skipCommand) {
-      // send the `endSessions` command
-      this.topology.endSessions(this.id);
-    }
-
     // mark the session as ended, and emit a signal
     this.hasEnded = true;
     this.emit('ended', this);