Remove last mentions of report processing

[edmorley]

There are a few places that still imply the report processing feature exists, even though it was removed in #30.

and a CSP report processing facility to Django.

-> https://github.com/mozilla/django-csp/blame/e5dc7f65b332d73c1ef8361500146e78bd282f88/README.rst#L5-L6

If you are not using the built in report processor

-> https://github.com/mozilla/django-csp/blame/e5dc7f65b332d73c1ef8361500146e78bd282f88/docs/installation.rst#L19-L20

It may also be worth mentioning on the "CSP Violation Reports" docs page that you'll now need to set up some other service for capturing the reports (and perhaps a link to https://developer.mozilla.org/en-US/docs/Web/Security/CSP/Using_CSP_violation_reports), if only since old blog posts about django-csp still mention the report processing feature, so this would make it much clearer.

I'll try to open a PR for this if I get a moment :-)