The legacy textbooks page has been replaced with an authoring MFE
equivalent. We don't need to keep the old one around.
This work is part of https://github.com/openedx/edx-platform/issues/36108
BREAKING CHANGE: With this change the `legacy_studio.textbooks` waffle
flag will no longer be respected and the system will behave as if the
flag is always set to False.
This basically commits the transpiled CoffeeScript JS (with minor
cleanup) and removes coffee build support.
A tiny amount of support for xblocks exists, because external users
may have xblocks with coffee. But no coffee in our tree anyway.