[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