gitignore: Forget the phantom path `coverage/`.

We generate a docs/_build/coverage/, and a var/coverage/
(and the latter is mapped as the URL `/coverage/` in dev),
but those are covered (ahem) by other ignore patterns
that ignore their respective parents.

.gitignore

@@ -35,7 +35,6 @@ npm-debug.log
 
 # Test / analysis tools
 .coverage
-coverage/
 
 ## Files left by various editors and local environments
 # (Ideally these should be in everyone's respective personal gitignore files.)