[sumac] fix: remove the "Legacy Library" button if libraries v1 are disabled [FC-0062]

[pomegranited]

Description

Backports #35736 to sumac.

Removes the "Legacy Library" button from the legacy Studio "new block" button array if Libraries v1 are disabled, either via waffle flag or via the ENABLE_CONTENT_LIBRARIES feature flag.

Supporting information

Addresses: openedx/modular-learning#237
Private-ref: FAL-3855

Testing instructions

See #35736

Deadline

ASAP

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