Remove the ASERT anchor walk and caching

Summary: After D11092 the axion activation (upgrade that enabled ASERT) is done from a block height. We can simply retrieve the anchor block from it's height without having to search for it based on time, and the cache is also no longer needed.

Test Plan:

ninja all check-all

Run IBD with no assume valid.

./contrib/teamcity/build-configurations.py lint-circular-dependencies

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11100