Remove Source/OWNERS

According to abarth, per-file doesn't actually work recursively, so this
file doesn't actually do anything. (In particular, it doesn't prevent
others than those listed from reviewing IDL changes.)

BUG=none

Review URL: https://codereview.chromium.org/266023004

git-svn-id: svn://svn.chromium.org/blink/trunk@173247 bbb929c8-8fbe-4397-9dbb-9b2b20218538