[token server + chronik-client + mock-chronik-client + ecash-herald] Remove legacy Dockerfile(s)

Summary: CI is now working for these apps/modules from the new top-level Dockerfiles

Test Plan: Confirm successful builds in CI (last builds for each of these apps was form new Dockerfile)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16169