Enable the `require-await` ESLint rule. Async functions without `await`
are just regular functions with extra overhead — marking them `async`
adds implicit Promise wrapping, can hide missing `return` statements,
and misleads readers into expecting asynchronous behavior where there is
none.
While fixing the violations, I removed unnecessary `async` keywords from
source files and from various test callbacks that never used `await`.
729f7f0fd1