Remove legacy v2 API tests which use wsgi_app()

There is fakes.wsgi_app() for testing legacy v2 API code, but the
method becomes unnecessary. So this patch removes the method and
unit tests which used the method for legacy v2 API.

NOTE: This patch just removes the test of createserverext extension
      which is not documented and not tested with Tempest.
      The extension is not implemented and seems unnecessary legacy.
      Let's remove the test.

Partially implements blueprint remove-legacy-v2-api-code

Change-Id: I0eaa0a9860af7d574a689722785d5d3d614eb0c1

Author: Ken'ichi Ohmichi