tests: remove duplicate block generation

Summary: I found that the test does not need the 6 extra blocks. This is probably a leftover from a partial backport form core tests, with Segwit tests removed.

Test Plan: test/functional/test_runner.py wallet_importdesc*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8999