Remove coveralls integration?

[RyanGlScott]

While scotty's ingration with coveralls.io is well intentioned, it's become unhealthy as of late:

• hpc-coveralls appears to be unmaintained: Allow building with Cabal-2.2 killy971/hpc-coveralls#73
• --enable-coverage support for cabal new-build is nonexistent, and this is unlikely to change any time soon: cabal new-test doesn't work with library-coverage for components with other-modules haskell/cabal#5213

In light of this, I think we should remove coveralls.io integration altogether for the time being (at the very least, until one of the two bullet points above changes).