Revert of [iOS] Defaults to use system libjpeg on iOS.

Revert of [iOS] Defaults to use system libjpeg on iOS. (patchset #1 id:1 of https://codereview.chromium.org/1806503002/ )

Reason for revert:
Chrome on iOS does not link against libjpeg (neither libjpeg_turbo nor libjpeg) and build using libjpeg_turbo when using gyp.

There is no path between //:gn_all and //third_party:jpeg, thus no need to select one version or the other on iOS. Revert the change as it causes link error for WebRTC.

$ gn path out/simulator //:gn_all //third_party:jpeg
No non-data paths found between these two targets.


Original issue's description:
> [iOS] Defaults to use system libjpeg on iOS.
> 
> It is currently not possible to build libjpeg_turbo on iOS (and may
> not be a good idea in term of application size) so default to using
> the version of libjpeg shipped with the system.
> 
> Move the "libs" assignment from the "jpeg" to "system_libjpeg_config"
> config.
> 
> BUG=459705
> 
> Committed: https://crrev.com/689dc0e45d16789f3ce3241d3033601af395c313
> Cr-Commit-Position: refs/heads/master@{#381432}

TBR=brettw@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=459705

[sdefresne, 2016-04-12 17:37:00]

Created Revert of [iOS] Defaults to use system libjpeg on iOS.

[commit-bot: I haz the power, 2016-04-12 17:37:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1882123002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1882123002/1

[sdefresne, 2016-04-12 17:39:13]

The CQ bit was unchecked by sdefresne@chromium.org

[sdefresne, 2016-04-12 18:08:21]

On 2016/04/12 at 17:37:29, commit-bot wrote:
> CQ is trying da patch. Follow status at
>  https://chromium-cq-status.appspot.com/patch-status/1882123002/1
> View timeline at
>  https://chromium-cq-status.appspot.com/patch-timeline/1882123002/1

Closing as the build can easily be fixed, see
https://codereview.chromium.org/1886443002.