This reverts commit 4f945599.

Reason for revert: Changed break Linux Bot:
https://ci.chromium.org/buildbot/client.v8/V8%20Linux%20-%20nosnap%20-%20debug/19064

Original change's description:
> Add Visitor for EphemeronHashTable
> 
> JSWeakCollection does not have any weak references anymore. Special
> handling of Ephemerons can now be implemented in
> VisitEphemeronHashTable.
> 
> Bug: chromium:844008
> Change-Id: I9f4d8ad6a32cc7a55b715803f6a83ff8d2743ce8
> Reviewed-on: https://chromium-review.googlesource.com/1090274
> Commit-Queue: Dominik Inführ <dinfuehr@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53584}

TBR=ulan@chromium.org,dinfuehr@google.com

Change-Id: Ifba31e75b0ca221da3a3905db829af586d342e26
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:844008
Reviewed-on: https://chromium-review.googlesource.com/1090971Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53588}