Delete Via 3 feature flag

**Confusingly this actually involves sending PRs to the LMS app**, to make it use Via 3, not sending PRs to Via 3. But the issue is in the Via 3 repo so that it can go in the milestone with other Via 3 issues.

- [x] Remove all references to the `use_via3_url` feature flag from the LMS code
- [x] There also appears to be a reference to a `use_via2_html_rewriter` feature flag in the LMS code, let's remove that too
- [x] Remove the `via3_url` setting from LMS `*.ini` files and code
- Remove envvars from LMS QA and prod in Elastic Beanstalk
- [x] `FEATURE_FLAGS_ALLOWED_IN_COOKIE`
- [x] `VIA3_URL`