This change drops the legacy studio custom pages UI aka. the tab edit
page.
This work is part of https://github.com/openedx/edx-platform/issues/36108
BREAKING CHANGE: The 'legacy_studio.custom_pages' waffle flag has been
removed and the code will work as if this flag is permanently set to
False.
Co-authored-by: Kyle McCormick <kyle@axim.org>