[tests] Remove mininode periodic (half-hour) ping messages

Summary:
Partial backport of core PR11712 (commit f2ae6f3)

Depends on D2356

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2357