test: remove redundant blocks generation logic

Summary:
those tests already have enough mature utxos from the pre-mined chain.

This is a partial backport of core#26886
https://github.com/bitcoin/bitcoin/pull/26886/commits/42029a7fd42e93426bdfb40d1918d739d20962af

Depends on D16025

Note that the interface_usdt_utxocache.py still needs blocks to be generated because we miss some backports (most likely culprit core#25528)

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16027