remove import for removed function and neuter unused caller

Django 1.7 removed `parse_backend_uri` which the redis connector requires.  There's a commit https://github.com/jbalogh/django-cache-machine/commit/5ab6389277e573af28ee9c0990df8b92a9055dfd in django-cache-machine, but there's no release that yet includes it, and there have been a lot of changes since then that we'd want to test carefully.

Fortunately the removed function is only used in the redis case, and we use memcache backend.  This patch just removes the import and call of the removed function.

@tgross removing this unused code that calls something we no long have seems like the minimally invasive change, and if this repo is any indication we're already running our own fork, right?