[cashtab, ecash-herald] Patch CI docker builds by removing now-deprecated ecash-script

Summary: We no longer need this dependency, nor do we have it available. CI is failing as still looking for it.

Test Plan:
docker build -f cashtab.Dockerfile -t cashtab_local .
docker build -f ecash-herald.Dockerfile -t ecash-herald_local .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18679