[webapi][XWALK-1940]Fix bug by removing show method for notification

- notification spec show() method have been removed.

Impacted TCs num(approved): New 0, Update 2, Delete 0
Unit test Platform: Tizen IVI
Andriod test result summary: Pass 2, Fail 0, Blocked 0
Merge pull request #335 from xinliux/xinliux/webapi/notification-1940

[webapi][XWALK-1940]Fix bug by removing show method for notification