Coveralls upload broken

Coveralls has not been deployed to since February. Apparently a token is needed:
```
ERROR: Coveralls submission requires a COVERALLS_TOKEN environment variable
```
Alternatively, we could just remove coveralls (codecov seems to work and I don’t see why we need *two* coverage trackers).

Inspect the repository's CI configuration and coverage-upload steps to find where Coveralls is invoked, then check how the existing Codecov upload works. Done means either configuring the required COVERALLS_TOKEN for successful Coveralls submissions or removing the Coveralls integration while retaining working coverage reporting.

Written by the indexing model from the issue text.