rand: remove getentropy() fallback for macOS < 10.12
rand: remove getentropy() fallback for macOS < 10.12

Summary:
We no longer support macOS < 10.12

This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/18364 | PR18364]]

Test Plan:
`ninja`

@bot build-osx

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8848

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