Remove c++20 header

Summary:
We don't enforce c++20 (yet) so we shouldn't include any c++20 header to avoid breaking the build on older supported compilers.
D17964 avoided the use of std::countr_zero because it's c++20, but the <bit> header was still included.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Subscribers: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D18081