[chronik] make -chronikelectrumbind require -chronikscripthashindex

Summary: Some Electrum RPCs (yet to be implemented) require the scripthash index, and imo it does not make sense to allow partial Electrum functionality. So make sure the index is available when the Electrum server is enabled.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17599