remove invalid form action "form_url"

When running with the django server locally with `TEMPLATE_DEBUG = True` and `TEMPLATE_STRING_IF_INVALID = 'INVALID_VARIABLE'` the import fails because the form action is set to `INVALID_VARIABLE`. I believe it is due to this line:

https://github.com/bmihelac/django-import-export/blob/b83aec0bc651f9e3e3a6b5498c20a9b2d312be43/import_export/templates/admin/import_export/import.html#L27

There is no `form_url` variable. In this pull request, I simply remove that variable so the form action is blank. I added some test code which failed before, but passes after this update.