chore: expand init instructions, clean after init

[markdalgleish]

This change was motivated by some upcoming work in create-remix but this is still a useful change in the meantime that can go out immediately.

From an end user perspective, if the remix.init script runs during the create-remix flow, the npx remix init instructions are no longer present in the readme since they're redundant and just add noise.

For those that didn't already run the remix.init script (either because they opted out of installing dependencies, or because they cloned this repo directly), the instructions are now much clearer since they're not conditional — you don't have to remember whether or not you opted out of installing dependencies anymore. We also now make it clear that this script makes changes to your project that are intended to be committed.

[markdalgleish]

This line isn't conditional in blues-stack, and there's actually no harm in running this script again. Since literally every contributor going forwards need to run this script, I think it's cleaner to leave it as just Initial setup: