Revert of [es6] ship Object.assign (patchset #2 id:20001 of https://codereview.chromium.org/1191003003/)

Reason for revert:
Seems to break asan somehow

Original issue's description:
> [es6] ship Object.assign
>
> BUG=v8:4007
> LOG=N
> R=rossberg@chromium.org, arv@chromium.org
> CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel
>
> Committed: https://crrev.com/12e194860a56d47a10d89ae34761a4af6b6166bb
> Cr-Commit-Position: refs/heads/master@{#29118}

TBR=arv@chromium.org,dslomov@chromium.org,rossberg@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4007

Review URL: https://codereview.chromium.org/1191593005

Cr-Commit-Position: refs/heads/master@{#29124}