Revert of Add d8 target to the GN build. (patchset #2 id:40001 of...

Revert of Add d8 target to the GN build. (patchset #2 id:40001 of https://codereview.chromium.org/834113005/)

Reason for revert:
V8 roll fails due to this change. (see https://codereview.chromium.org/843673003/)

Original issue's description:
> Add d8 target to the GN build.
>
> Also formats the rest of the file with "gn format". This accounts for all the changes except for the "d8" target additions.
>
> Committed: https://crrev.com/36383f08c1f692ea797d54a27d7c6b3d3d53b440
> Cr-Commit-Position: refs/heads/master@{#25984}

TBR=jochen@chromium.org,machenbach@chromium.org,brettw@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/838983002

Cr-Commit-Position: refs/heads/master@{#25985}