[iOS] Remove kkhorimoto from ios/build/bots/OWNERS

Bug: none
Change-Id: I12c62918c78a9cd8249396b6cfe33a5bb544c254
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084714
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746455}

ios/build/bots/OWNERS

 justincohen@chromium.org
-kkhorimoto@chromium.org
 rohitrao@chromium.org
 sdefresne@chromium.org
 sergeyberezin@chromium.org