Fix chromiumos breakage for gn_all.

Fix chromiumos breakage for gn_all.

The gn_all target in all.gyp is attempting to build ':image_diff#host',
which I think may not work correctly in a chromeos==1 build. I think
the '...#host' syntax should probably only be used on the android port
(looks like that's what content_shell.gypi does). Though, given that
we're building content_shell, we shouldn't need the image_diff
dependency at all, so I'm just deleting it outright.

TBR=abodenha@chromium.org, brettw@chromium.org, hshi@chromium.org
NOTRY=true
NOTREECHECKS=true
BUG=461602

Review URL: https://codereview.chromium.org/951553004

Cr-Commit-Position: refs/heads/master@{#317953}