[chronik] Remove the legacy NNG tests
Summary: These tests rely on a live NNG server which has been shut down and are no longer working. Since we are deprectating NNG support, just remove the tests.
Test Plan:
./contrib/teamcity/build-configurations.py chronik-client-tests
Reviewers: #bitcoin_abc, tobias_ruck
Reviewed By: #bitcoin_abc, tobias_ruck
Subscribers: tobias_ruck
Differential Revision: https://reviews.bitcoinabc.org/D16572