[chronik-client] Deprecate client-side ping keepAlive
Summary: Server-side ping is available in NNG chronik and under review for in-node chronik. The client side implementation is obsolete.
Test Plan: npm test
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D15070