Remove report-a-problem from 4XX pages

We shouldn't make users fill this out:

- this form captures info that we already know (from fastly or GA)
- this feedback isn't really being used in a consequent manner

This has been agreed with Roo.
Merge pull request #421 from alphagov/remove-report-a-problem-from-4XX-pages

Remove report-a-problem from 4XX pages