logging: Drop BCLog::Level::None

Summary:
Now that Info-level logging is always logged, there is no further need for the "None" level, so remove it.

This is a partial backport of core#28318
https://github.com/bitcoin/bitcoin/pull/28318/commits/667ce3e3297645527b07314e1d5a82275fb25845
Depends on D18271

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18272