The following code was previously accepted:

  async function f() {
    let g = (await) => {};
  }

But per the spec, using 'await' is disallowed in arrow parameters
by an early error rule (just as 'yield' is disallowed in arrow
params inside generators).

There was special logic in ParseUnaryExpression which seems to have been
there only to allow that case. Having removed it, we get a SyntaxError in
the right cases anyway when ParseUnaryExpression chokes on whatever
illegal token follows 'await' in the cases this code previously handled.

Also removes the unnecessary AsyncBindingPatternProduction enum value.

R=caitp@igalia.com, littledan@chromium.org
BUG=v8:4483

Review-Url: https://codereview.chromium.org/2258313002
Cr-Commit-Position: refs/heads/master@{#38802}