```
On the ::SLOW or ::SLEEP paths, we would feed our RNG output back into
OpenSSL using RAND_add. This commit removes that functionality.
RAND_add(): https://www.openssl.org/docs/manmaster/man3/RAND_add.html
RAND_add() mixes the num bytes at buf into the internal state of the
random generator. This function will not normally be needed, as
mentioned above. The randomness argument is an estimate of how much
randomness is contained in buf, in bytes, and should be a number
between zero and num.
```
Partial backport of core PR17265:
https://github.com/bitcoin/bitcoin/pull/17265/commits/5624ab0b4f844dc7c17aeb1b009f002c33c38fb3