remove obsolete DM test configs

ViaSingletonPictures simulates a load pattern that Blink no longer produces.

I don't even remember what Via2ndPic and ViaTwice are meant to test,
but I imagine they're quite obsolete.

There was a lingering reference to ViaMojo, which otherwise doesn't exist.

Bug: skia:7544

Change-Id: I033fd344314054ad66c363e5dbd0a373be188069
Reviewed-on: https://skia-review.googlesource.com/88780
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
13 files changed