Obsolete invocations of refresh_drupal_themes

Related: ckanext-spatial#1

The functions refresh_drupal_themes and refresh_drupal_themes_async defined in ckanext/dgu/lib/helpers.py are obsolete in absence of Drupal, but are still called by

ckanext/dgu/logic/action/update.py
ckanext/dgu/controllers/package.py

as well as by ckanext-spatial in

ckanext-spatial/ckanext/spatial/harvesters/opsi.py

This causes errors such as the following, which occurs during harvesting:

ERROR [ckanext.dgu.lib.helpers] ERROR occured while signaling drupal to refresh themes: <urlopen error [Errno -2] Name or service not known>

As part of the migration away from Drupal, these functions and their invocations should be removed.