Step down as core/ OWNER

Step down as core/ OWNER. I am moving on to new things and I will no longer be
able to be as active as I was in Blink. It is best to remove my email from
OWNERS files to avoid confusion.

R=eseidel@chromium.org, adamk@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/493753002

git-svn-id: svn://svn.chromium.org/blink/trunk@180667 bbb929c8-8fbe-4397-9dbb-9b2b20218538