ozone: Remove the "drm" software composited platform
This is a software-only platform that was used early in the process
of bringing up the Chrome OS ozone/freon (no X11) build. It's time to
remove it because it now requires not only scary flags to get it going,
but also a patch (https://codereview.chromium.org/
1216303004/).
If we need a build with software rasterization let's use vgem+swrast like
our VMtests do.
BUG=531814
TEST=link still works fine with platform=gbm
Review URL: https://codereview.chromium.org/
1345703002
Cr-Commit-Position: refs/heads/master@{#349012}