Revert of Clean up the top level BUILD.gn file. (patchset #2 id:20001 of https:/...

Revert of Clean up the top level BUILD.gn file. (patchset #2 id:20001 of https://codereview.chromium.org/963093003/)

Reason for revert:

ERROR at //BUILD.gn:246:7: Item not found
      "//third_party/usrsctp",  # TODO(GYP)
      ^----------------------
You were trying to remove "//third_party/usrsctp"

Original issue's description:
> Clean up the top level BUILD.gn file.
>
> There was a bunch of todo-related stuff that could
> be cleaned up, and this change also removes the 'gyp_all'
> target which was mostly just duplicating stuff in all.gyp.
>
> This CL should have no actual affect on the builds, apart
> from deleting the one target, but is also a prelude to
> subsequent CLs for this bug.
>
> R=brettw@chromium.org
> BUG=461019
>
> Committed: https://crrev.com/18e5743bbe39f30b75abad4c5c7f0efab288ed9a
> Cr-Commit-Position: refs/heads/master@{#318523}

TBR=brettw@chromium.org,dpranke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=461019

Review URL: https://codereview.chromium.org/962383002

Cr-Commit-Position: refs/heads/master@{#318527}