Bug: Hide Legacy Library content button in legacy Studio when Libraries v1 are disabled

[pomegranited]

Legacy Library green button doesn't disappear when you disable legacy libraries, whether by waffle flag or the big ENABLE_CONTENT_LIBRARIES feature flag.

[pomegranited]

@jmakowski1123 @lizc577 @sdaitzman @marcotuts This is ready for AC testing on the sandbox. Please see the Testing Instructions on openedx/openedx-platform#35736 for how to test this -- it requires Django Admin access to change some waffle flags.

[ChrisChV]

@jmakowski1123 @lizc577 @sdaitzman @marcotuts Friendly reminder that this is ready for AC testing on the sandbox.