random: Remove remaining OpenSSL calls and locking infrastructure

Summary:
Partial backport of core PR17265:
https://github.com/bitcoin/bitcoin/pull/17265/commits/b49b6b0f7090cc15860d815fb0ef306ddfc718ba

Depends on D6240 and D6241.

Test Plan:

ninja all check-all

Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6242