Removes myself from a couple of OWNERs files

R=jam@chromium.org, oshima@chromium.org
TBR=jam@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/692863002

Cr-Commit-Position: refs/heads/master@{#302079}

athena/OWNERS

 mukai@chromium.org
 oshima@chromium.org
-sky@chromium.org

content/browser/OWNERS

-sky@chromium.org
-
 per-file child_process_security_policy_impl.*=tsepez@chromium.org
 per-file child_process_security_policy_impl.*=creis@chromium.org
 per-file child_process_security_unittest.cc=tsepez@chromium.org