Remove the duplicated external extension example.

The whole contactextension is not what we developed for W3C SysApps, it
is just an external extension example. It is dup with same dir
myextension. Besides, the myextension is protected by RuntimeClient and
RuntimeEmbededClient instrumentation test for each build. So we decide
to remove this contact example.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2539