Removing ex-contributors from ash/wm/OWNERS

Personally, I no longer feel confident to review code in ash/wm

BUG=None

Change-Id: I5258309ba7171eb34ad3a3b9b78ca2bb6cdc71f2
Reviewed-on: https://chromium-review.googlesource.com/783800Reviewed-by: Valery Arkhangorodsky <varkha@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521045}

ash/wm/OWNERS

-pkotwicz@chromium.org
-varkha@chromium.org
 flackr@chromium.org
 
 # COMPONENT: UI>Shell>WindowManager