polish: remove misleading comment (graphql#3806)

We could not demonstrate via actual benchmarking that the tick reduction
from awaiting a promise prior to returning it translates into real-world
performance gains. See graphql#3796.

In any case, the promise must be awaited (irrespective of performance)
to allow the catch block to handle rejection errors. Simply returning
`completed` would result in test failures.

Author: yaacovCR

src/execution/execute.ts

@@ -942,8 +942,6 @@ async function completePromisedValue(
       asyncPayloadRecord,
     );
     if (isPromise(completed)) {
-      // see: https://github.com/tc39/proposal-faster-promise-adoption
-      // it is faster to await a promise prior to returning it from an async function
       completed = await completed;
     }
     return completed;