frontend: remove dependency coveralls

We anyway run coveralls via npx in the CI.
Coveralls did not receive updates for some time.
Coveralls.io wants to move to the universal coverage reporter
(https://docs.coveralls.io/integrations#official-integrations).
The npm package has some dependencies which have moderate
security issues:
https://github.com/ecamp/ecamp3/security/dependabot/165
https://github.com/ecamp/ecamp3/security/dependabot/153