Remove --experimental-local middleware for pretty-error pages

[mrbbot]

See cloudflare/miniflare#436 for an explanation of why we needed to add this. This issue is just here to remind us to remove this middleware when we no longer need it.

[penalosa]

Closing this issue for now, since even if we can remove the need for it in Wrangler through workerd improvements we'll still need to make use of it in the dashboard and playground