test: remove assert_blockchain_height

Summary:

This simplifies the code and solves intermittent timeouts caused by commit rABC0c0123b0aa87

Note: I have not yet seen an intermittent timeout happen, but the conditions for this to happen exist after D10896

This is a backport of core#21117

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10950