Remove hwrand_initialized.

Summary:
All access to hwrand is now gated by GetRNGState, which initializes the hwrand code.

Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/4ea8e50837a0932b31a241988fd68d6730a2048a

Depends on D4396

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4397