Remove tests run against Storybook v5

It's difficult to set up a Webpack alias from 'storycap' to
'@remix/storycap' in Storybook v5, and tinkering with the presets.js
file (the precursor to main.js) is yielding little fruit. Since we don't
care about Storybook v5 support in the Remix monorepo anyways, given
that we're already on v6, we just remove the v5 tests in this repo.