rand: remove getentropy() fallback for macOS < 10.12

Summary:
We no longer support macOS < 10.12

This is a backport of Core 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