test: remove TODO in feature_assumeutxo.py

Summary: Done in D17999. m_best_header is now correctly recalculated after the invalidateblock + reconsiderblock operation that is done in dumptxoutset

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18067