chore: remove md4 and related ENABLE_BLAKE2B_HASHING feature flag

[Anas12091101]

Description

This PR removes md4 and related ENABLE_BLAKE2B_HASHING feature flag

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Supporting information

#36024

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"None"

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[openedx-webhooks]

Thanks for the pull request, @Anas12091101!

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[feanil]

Changes look good, we can merge after the DEPR is accepted coordinating with anyone that we need to coordinate with.