Remove agable, bradleynelson from //OWNERS.

bradleynelson is no longer active in depot_tools. agable@ is
unavailable for the rest of 2018.

Change-Id: Idf5be1db84b8f6857f9edd7ed16c56d4c89f81ef
Reviewed-on: https://chromium-review.googlesource.com/1256170Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>

OWNERS

 set noparent
-agable@chromium.org
-bradnelson@chromium.org
+
+# unavail for rest of 2018
+# agable@chromium.org
+
 dpranke@chromium.org
 ehmaldonado@chromium.org
 hinoka@chromium.org