Disable SkTaskGroup in SkMultiPictureDraw temporarily.

Think this should be enough to get rolling again.

NOTREECHECKS=true

Review URL: https://codereview.chromium.org/687273006
Revert harder, removing SkTaskGroup.cpp from core temporarily.

Not all linkers are seeing the SkThread code is not needed.

BUG=skia:
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/673903007