Function.prototype.toMethod was removed from ES6.

This removes the function and updates the tests to either
use %ToMethod or a dedicated syntax (using concise method
or a class).

BUG=v8:3330
LOG=N
R=dslomov@chromium.org, adamk

Review URL: https://codereview.chromium.org/914713002

Cr-Commit-Position: refs/heads/master@{#26559}