In ES2016, the Proxy enumerate trap is removed. This patch changes
for-in iteration on Proxies to use the ownKeys trap. Due to the clean
organization of that code, the patch basically consists of deletions.

R=adamk
LOG=Y
BUG=v8:4768

Review URL: https://codereview.chromium.org/1717893002

Cr-Commit-Position: refs/heads/master@{#34200}