chore: remove unused coveralls dependency

[shadowspawn]

There are multiple warnings during npm install in this repo about stale dependencies that turn out to be due to coveralls.

For example:

$ npm install
...
npm WARN deprecated har-validator@5.1.5: this library is no longer supported


$ npm ls har-validator
yargs@17.7.2 /Users/john/Documents/Sandpits/yargs/my-yargs
└─┬ coveralls@3.1.1
  └─┬ request@2.88.2
    └── har-validator@5.1.5

coveralls was removed from the coverage workflow in #1578.

So remove coveralls as a dependency.

[bcoe]

Did this in my cleanup of dependencies to get build passing.