Revert of [esnext] ship String.prototype.padStart / String.prototype.padEnd (patchset #1 id:1 of https://codereview.chromium.org/2382193002/ )

Reason for revert:
Causes several GC bugs, e.g. https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20arm64%20-%20sim%20-%20gc%20stress/builds/2858/steps/Mjsunit/logs/debug-constructed-by

Original issue's description:
> [esnext] ship String.prototype.padStart / String.prototype.padEnd
>
> Enable the --harmony-string-padding flag by default
>
> BUG=v8:4954
> R=adamk@chromium.org, littledan@chromium.org
>
> Committed: https://crrev.com/8352a0feaccfd9a19f3b38564ed2c0859dd6e3d5
> Cr-Commit-Position: refs/heads/master@{#40060}

TBR=littledan@chromium.org,adamk@chromium.org,foolip@chromium.org,caitp@igalia.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4954

Review-Url: https://codereview.chromium.org/2398183003
Cr-Commit-Position: refs/heads/master@{#40071}