[application] Remove "application launch from local path" support

We rely fully on main document as application entry point (see
http://anssiko.github.io/runtime/app-lifecycle.html).

The deprecated "application launch from local path" code path should be removed as
1) it is not used but brings extra complexity
2) it does not provide the logic which we rely on in many places within xwalk (for instance initialization of main_runtime_ or non-persistent main document implementation).