Collectstatic failed in production when comprehensive theme contained custom css files.
This patch fixes that problem by removing ComprehensiveThemeFinder from STATICFILES_FINDERS
and ComprehensiveThemingAware mixin from STATICFILES_STORAGE.

Comprehensive theme static dirs are added to the top of the STATICFILES_DIRS entry,
which means that the default django FilesystemFinder will find theme static files,
and since the theme folder is at the top of STATICFILES_DIRS, theme files will take
precedence over default LMS/CMS static files.

This change means that theme static file URLs are no longer prefixed with themes/<theme-name>/,
but since we currently only support one comprehensive theme at a time, that shouldn't be a problem.
If/when we want to make the choice of a theme dynamic per-request (microsites?), we will have to
bring custom theme finders and storage mixins back, but for now, we don't need them.