[chronik] Electrum: consistently avoid repeating the serde_json namespace

Summary:
This is already imported and there is no name conflict, so keep it simple.

Depends on D18105.

Test Plan:

ninja

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18106