[e.cash] Remove uneeded halving banner

Summary:
The halving blockheight banner has served its purpose and currently flashes for a moment when loaded
Removing the uneccessary code and api call

Test Plan: Preview the site and check the other banner still looks good and the halving one is gone

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15964