Remove chapter about errbit from "Retiring an application"
Remove chapter about errbit from "Retiring an application"

Removing this without a replacement because I'd like to keep the errors
in Sentry, even if the application has been retired. It might give us
extra historical context when looking at pull requests and Trello
tickets, and doesn't cost us anything.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs