[tests] Remove unnecessary cs_mains in denialofservice_tests

Summary:
9fdf05d70cac4a62d1aeeb4299e2c3a9a866f8af (D3606) resolved some lock
inversion warnings in denialofservice_tests, but left in a number
of cs_main locks that are unnecessary (introducing lock inversion
warnings in future changes).

This is a backport of core#19533

Test Plan:
with TSAN:
ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9689