Can we remove setup process for `icu4c`?

[jagthedrummer]

This is a native dependency needed by charlock_holmes, but as of bullet-train-co/bullet_train-core#1002 we don't require charlock_holmes by default.

That's a gem that's just needed by bullet_train-conversations and it does nothing but cause problems in a fresh BT app.

We can probably remove the bits of the configure/setup scripts that look for icu4c, though we'll want to add some details to the README for the bullet_train-conversations gem that help people get it installed.