migrate-from-govuk-lint: Delete doc
migrate-from-govuk-lint: Delete doc

- We've migrated all the things that we own. It's been six months since
  this was last reviewed, I think we can say it's all done now. This
  doesn't say anything more than the README of each project says anyway.

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