Remove Cypress tests and other unused files and dependencies

[36degrees]

The Cypress tests are currently failing for pull requests against the main branch, seemingly because they’re running against an old version of the Prototype Kit which does not support GOV.UK Frontend v5.0.

The tests are also designed to test functionality of the Prototype Kit itself (rather than the docs site) and are largely (possibly entirely) duplicated in the Cypress tests in the Prototype Kit repo from which this repo was forked.

Given this duplication (and the fact that the tests run against an old version of the Prototype Kit), this PR:

• removes the Cypress tests
• removes the related npm scripts and GitHub workflow step
• uninstalls Cypress, including cleaning up its config file and other references across the repo

Additionally, it also:

• removes an unused test utils file (none of the members exported from this file are used by any other file – looks like the last references to them were removed in https://github.com/alphagov/govuk-prototype-kit-docs/commit/77132eae91bc065fea26bafb1a5a48b61eb7300f and https://github.com/alphagov/govuk-prototype-kit-docs/commit/9ee73489031432871f32b0416452d8da76b5b903)
• removes the notifications-node-client dependency – this is only needed for people building prototypes who want to use Notify and is not needed for the docs site