Remove redundant p2p lock tacking for tx download functional tests

Summary:
New functional test coverage of tx download was added by #19988,
but with p2p_lock is redundant for some tests with wait_until
test helper, already guaranteeing test lock tacking.

This concludes backport of core#20171
https://github.com/bitcoin/bitcoin/pull/20171/commits/bc4a23008762702ffcd6868bcdb8fe2a732640ba
Depends on D18653

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18654