tools/gn: Remove fallback to direct_dependent_configs.

tools/gn: Remove fallback to direct_dependent_configs.

If public_configs were undefined we would fallback to
direct_dependent_configs.

we can remove this now, since we converted all the projects to
public_configs already.

BUG=None
R=brettw@chromium.org

Committed: https://crrev.com/5591967b16af40155e7feb77efd6410b6c94a4ef
Cr-Commit-Position: refs/heads/master@{#346769}

[brettw, 2015-09-01 22:09:44]

lgtm

[tfarina, 2015-09-01 22:54:07]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-09-01 22:54:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1307223009/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1307223009/1

[commit-bot: I haz the power, 2015-09-01 23:01:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-01 23:02:17]

Patchset 1 (id:??) landed as
https://crrev.com/5591967b16af40155e7feb77efd6410b6c94a4ef
Cr-Commit-Position: refs/heads/master@{#346769}