rootdev: remove xiaochu@ from OWNERS

He's left the team.

BUG=chromium:995893
TEST=None

Change-Id: I2909e5ee4e4cf128c05659314d5c0ae7e0bb84a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/rootdev/+/2001410
Tested-by: Nicolas Norvez <norvez@chromium.org>
Auto-Submit: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
Commit-Queue: Nicolas Norvez <norvez@chromium.org>