Remove platform/drm files.

The code in platform/drm contains minimal functionality now and can
be removed.

BUG=224791
TEST=EME content tests pass

Review URL: https://codereview.chromium.org/186093003

git-svn-id: svn://svn.chromium.org/blink/trunk@168464 bbb929c8-8fbe-4397-9dbb-9b2b20218538