chore: remove src/api.ts (#6191)

Now the async hooks helper is gone api.ts was only used by the coverage
tools and by doclint.

DocLint is nearing the end of its lifespan with the TSDoc work, so I
focused on how best to define a list of modules for the coverage
tooling. They define an object of classes, and the path to that module.
They need the full path because we also check if the module exports any
events that need to be emitted - the coverage tool asserts that the
emitting of those events is also tested.

It's not _great_ that DocLint relies on a constant defined in the
coverage utils, but it should only be this way for a short period of
time and no one is actively working on DocLint (bar the effort to remove
it) so I don't think this is worth worrying about.

This change also broke the DocLint tests; based on the fact that DocLint is on its way out it doesn't feel worth fixing the tests, so this commit also removes them.

.travis.yml

@@ -65,7 +65,6 @@ jobs:
       script:
         - npm run compare-protocol-d-ts
         - npm run lint
-        - npm run test-doclint
         - npm run ensure-new-docs-up-to-date
 
     # This bot runs separately as it changes package.json to test puppeteer-core