Revert "Split Valgrind jobs -- one DM/NB run per task."

Revert "Split Valgrind jobs -- one DM/NB run per task."

This reverts commit bc9cd524d8ce8fd61ff6e49fa2a6e2a26a069874.

Reason for revert: Unable to find Valgrind binary.

Original change's description:
> Split Valgrind jobs -- one DM/NB run per task.
>
> Remove special build for Valgrind -- Valgrind uses the unmodified binary. See skia:6267.
>
> BUG=skia:2789,skia:6267
>
> Change-Id: Id56c460abf36eb2be633ac3a309e0dbc7a278f41
> Reviewed-on: https://skia-review.googlesource.com/8723
> Commit-Queue: Ben Wagner <benjaminwagner@google.com>
> Reviewed-by: Eric Boren <borenet@google.com>
> Reviewed-by: Mike Klein <mtklein@chromium.org>
>

TBR=mtklein@chromium.org,borenet@google.com,mtklein@google.com,benjaminwagner@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:2789,skia:6267

Change-Id: I16876a8528c441805931ed21e9aa8688ab2e0b4c
Reviewed-on: https://skia-review.googlesource.com/8831
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>