[electrum] use existing util.get_interfaces function for get_merkle_root

Summary: Remove some code duplication to keep maintenance of basic functionality common for all scripts.

Test Plan: python electrum/scripts/get_merkle_root.py -v

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15397