Remove virtual/softwarecompositing suite

Layout tests shouldn't be used to test compositor features.  The cc
pixel tests already include a large number of pixel tests for non-GPU
composited output that cover the set of cases that are required.

Additionally, non-gpu canvas is already tested via the fast/canvas
non-virtual suite.

R=abarth@chromium.org,senorblanco@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/473473003

git-svn-id: svn://svn.chromium.org/blink/trunk@180387 bbb929c8-8fbe-4397-9dbb-9b2b20218538